3 Scientific conclusions of Working
Group I contribution to the Fifth Assessment Report
Overall
impression
34. A very large number of the respondents to our
inquiry, most representing eminent scientific establishments,
praised the WGI contribution to AR5 as being an authoritative,
comprehensive and robust analysis of the past, present and likely
future states of the climate.[71]
Many highlighted the consistency of conclusions with previous
reports and growing evidence base.[72]
Professor Sir Brian Hoskins, Director of the Grantham Institute
for Climate Change at Imperial College London and a former IPCC
author, told us:
After more years of research and observations,
there is not a huge change in the conclusions of the AR5 report
and the science compared with the previous reports. [
] The
main thing is there is a continued and stronger message that we
can see the changes in the climate system that are consistent
with the increase in greenhouse gases [
]. Understandings
have increased and models have become more complex, but still
the same sorts of conclusions ride through as were there in the
previous reports.[73]
Dr Stott, head of the Climate Monitoring and Attribution
team at the Met Office and IPCC lead author, suggested that the
WGI contribution to AR5 supported the case for tackling climate
change:
You will see further evidence to support the
conclusion that warming is unequivocal and then we have more evidence
from right across the climate system of the effects of human influence
on the observed changes. That is not just in temperatures of the
atmosphere. It is also in temperatures of the ocean, reductions
in snow and ice, rising sea level, and changes in some climate
extremes. We have more evidence that strengthens that assessment
on human influence on climate and then we also have new evidence
around the future, the conclusion that continued emissions of
greenhouse gases will cause further warming and further changes
in the climate system. Limiting climate change will require substantial
and sustained reductions of greenhouse gas emissions.[74]
There were, however, a number of scientific conclusions
reported by the IPCC that have received criticism from both inside
and outside the scientific community. This chapter will examine
those criticisms in more detail.
Causes of climate change
35. The science of establishing the causes of climate
change is known as "attribution" and is dealt with explicitly
in Chapter 10 of the WGI contribution to AR5. The IPCC concluded
that it is "extremely likely" that more than half of
the observed increase in global mean surface temperature (GMST)
from 1951 to 2010 was caused by human influence, primarily through
increases in greenhouse gases.[75],
The resulting impact of this human-caused surface warming
on the atmosphere, oceans (heat content, sea-level, acidification,
salinity etc.), global water cycle, global ice-systems and extreme
weather events is discussed in the WGI contribution to AR5. The
phrase "extremely likely" has a 95% level of certainty
associated with it; an increase from the 90% level of certainty
attached to a similar statement found in AR4 (which was released
in 2007).[76]
36. The Grantham Institute for Climate Change at
Imperial College London stated that there is "no real question
that the climate is changing and that a significant part of these
changes is due to human greenhouse gas emissions".[77]
We have found no credible scientific source that disputes the
claim that human activity has an influence on the climate. There
are, however, a small number of scientists who dispute the extent
to which human influence is the dominant factor in recently observed
climate changes.[78]
Richard Lindzen, Alfred P. Sloan Professor of Meteorology at Massachusetts
Institute of Technology (MIT) and a former IPCC lead author, for
example, did not agree with the IPCC's conclusion, claiming that
"the attribution of most of the small warming to man is faulty
and inconsistent".[79]
Professor Lindzen and others allege that the IPCC overlooked natural
factors such as internal variability and the influence of the
sun (through cosmic rays) as considerable contributors in the
explanation of the warming trends (see paragraph 39).[80]
Additionally, some have queried the scientific justification for
assigning a 95% confidence level to the claims about the human-influence
on global warming since 1951, and for the IPCC attribution arguments
in general.[81]
37. The IPCC process for attributing the causes of
climate change was well-received by the majority of the scientific
community. In this process, the relative size, timing and spatial
pattern of phenomena which might influence whether the earth warms
or cools (known as climate forcing and including, for example,
greenhouse gas emissions) is matched against observed climate
change in a method known as "fingerprinting".[82]
This was explained in detail by Dr Shuckburgh, Head of the Open
Oceans research group at the British Antarctic Survey and Fellow
of the Royal Meteorological Society:
There are particular patterns that one anticipates,
being the pattern of change that you would expect from increasing
greenhouse gases. That is a pattern of the change over time and
the change in space. The change in space is both the geographical
pattern of the change and the change with altitude as you go up
through the atmosphere.
There are particular patterns that one can identify
that would be characteristic of the changes to the climate system
you would anticipate from increasing greenhouse gases and that
you would anticipate from natural fluctuations. For example, the
most well-known natural fluctuation, the El Niño phenomenon,
is associated with particular temperature changes, particularly
in the Pacific Ocean. There is a particular pattern that one identifies
with the El Niño phenomenon. If you look to the pattern
of temperature change geographically, which one would anticipate
from increasing greenhouse gases, it is different.
You identify those patternsthose are the
fingerprintsand then you can look to the observations of
what has occurred over the recent time history and disentangle
how much of a contribution has come from one pattern and how much
of a contribution has come from another pattern. That is the scientific
basis for formulating those statements.[83]
38. In establishing the clear pattern of human-influence,
the importance of ruling-out of the so-called "null hypothesis"[84]
was also emphasised, particularly by Dr Peter Stott:
We, of course, checked out this null hypothesis;
that we could explain the observed changes purely by these types
of internally generated variability [
] It is fully factored
into the assessment where we conclude that the dominant cause
of the warming since the mid-20th century is anthropogenic
and not these natural factors.[85]
[
] Our null hypothesis starting point is
that we can explain it just by internal natural processes and
we can rule that out at very high confidence levels.[86]
The importance of the time-scale was made very clear
when distinguishing between human influences and natural variability,
as stressed by Professor Sir Brian Hoskins:
[Climate change] is not all natural and it is
not all anthropogenic. It is a mixture. What we have on all timescales
is a mixture and this makes it difficult to understand and a very
interesting scientific problem, but on a day-to-day basis it is
dominated by natural variability [
] It is disentangling
these two, but with the very strong theoretical basis and the
observed warming over a 100-year period giving some agreement
we are seeing what we think is the fingerprint of our anthropogenic
warming on that century timescale. When you get down to the decadal
timescale, then we see a lot of the natural variability tending
to dominate.[87]
39. In addition to internal variability and changes
in the brightness of the Sun as experienced by the Earth (Total
Solar Irradiance), the IPCC also considers the plausibility of
cosmic rays playing a role in influencing global cloud cover (and
thus surface temperatures). The IPCC concluded that the link between
cosmic rays and cloud properties did "not prove to be robust
on the regional or global scale" despite the fact that "there
has been [a] hypothesis for decades".[88]
Professor Lindzen pointed out that this field is subject
to ongoing experiments.[89]
40. The slight increase in confidence in the statement
of attribution (95% in AR5 compared to 90% in a similar statement
in AR4) was made, in part, on the basis of the ability of the
IPCC to draw upon a "larger body of refereed literature".[90]
Professor Brigitte Nerlich and Dr Luke Colins from the University
of Nottingham were critical that there was minimal elaboration
of how the 95% figure was achieved, despite the recommendations
of the InterAcademy Council.[91]
Dr Stott, however, provided the justification for this increased
confidence:
Previously in the AR4 there was a lot of variability
apparently in the observational records, which we have now understood
was an artefact of a different bias in those measurements. That
is one example of where we have improved our confidence, but it
is not just in terms of ocean temperatures. It is also in terms
of other factors such as the retreating ice and snow. We have
developed our understanding of the water cycle. We have developed
our understanding of extremes. When you look into our chapter
10, you will see we have a table at the back there with 33 rows
where we have itemised all the evidence, both the evidence where
we have multiple lines of evidence and also where we have remaining
uncertainties. We have a greater wealth of evidence now than we
had, with better understanding, with more observations and also
with the improved models as well.[92]
41. There has been some confusion about the IPCC's
use of (un)certainty language.[93]
Professor Brigitte Nerlich and Dr Luke Colins explored the language
used to convey the IPCC's conclusions in the media. They argued
that conveying scientific information, including information about
levels of (un)certainty, is difficult because scientists and the
general public understand this word differently.[94]
James Painter, Head of the Journalism Fellowship Programme at
the Reuters Institute of Journalism drew a comparison between
the IPCC "95% certain" attribution statement and the
"95% certain" statement made by scientists concerning
the link between smoking and lung cancer.[95]
Professor Brigitte Nerlich and Dr Luke Colins argued, however,
that analogies could be misleading.[96]
42. The WGI
contribution to AR5 presents the most compelling evidence to date
that many of the changes to the climate recorded in the latter
half of the 20th Century were driven by post-industrial
human activity. We have not found convincing evidence that challenges
the IPCC's conclusion in this matter. There is increased confidence
in the IPCC projections that, with rising greenhouse gas concentrations,
we will continue to see warming (and the changes to the climate
associated with warming) in this century and beyond.
Sensitivity of the climate to
carbon dioxide
43. In order to ascertain the potential impact of
human-influence on the climate system, it is crucial to understand
how sensitive the surface temperature of the Earth is to changes
in the concentration of carbon dioxide in the atmosphere. The
IPCC reports on two important parameters that reflect this relationship:
the longer-term Equilibrium Climate Sensitivity (ECS),[97]
and the shorter-term Transient Climate Response (TCR).[98]
The two are related, but differ slightly in their definitions
so that the TCR is generally lower than the ECS. They can be calculated
from combinations of model simulations, observations and references
to the historical climate record. On ECS, the WGI contribution
to AR5 concluded:
Equilibrium climate sensitivity is likely in
the range 1.5°C to 4.5°C (high confidence), extremely
unlikely less than 1°C (high confidence), and very unlikely
greater than 6°C (medium confidence). The lower temperature
limit of the assessed likely range is thus less than the 2°C
in the AR4, but the upper limit is the same. This assessment reflects
improved understanding, the extended temperature record in the
atmosphere and ocean, and new estimates of radiative forcing.
[
] No best estimate for equilibrium climate sensitivity
can now be given because of a lack of agreement on values across
assessed lines of evidence and studies.[99]
On TCR, the WGI contribution to AR5 concluded:
The transient climate response is likely in the
range of 1.0°C to 2.5°C (high confidence) and extremely
unlikely greater than 3°C.[100]
The assessed ranges for these values have remained
very similar with each successive IPCC Assessment Report, prompting
Professor Sir Brian Hoskins to comment that:[101]
I think one of the amazing things about the IPCC
is how the equilibrium climate sensitivity range has stayed the
same over the years. That would seem we have not been doing any
science to try and refine this, but during that period those started
with an atmosphere-only model with a fixed ocean underneath. Then
gradually the system has become the whole depth of the ocean involved,
the vegetation on the land and the ice. The system has become
so much more complex and interactive that is modelled and uncertainty
ranges could well increase as you do that. The triumph has been
that most of them have stayed about the same.[102]
44. Most, but not all, of the discussions on the
IPCC treatment of sensitivity has related to the ECS. Professor
Lindzen and Nicholas Lewis, a climate researcher and mathematician,
both argued that the figure in WGI contribution to AR5 is too
high. The IPCC's slight increase in uncertainty of the figure
was also criticised.[103]
However, Professor Myles Allen, Professor of Geosystem Science
at the University of Oxford and former IPCC Lead Author, who took
a leading role in the IPCC discussions of ECS urged that the focus
should be on the TCR which, he argued, was a better indicator
of the warming expected over the coming century.[104]
This point was repeated by a number of others, including Mr Lewis,
who told us that "for policy purposes, the transient climate
response, which is approximately over a 70-year period during
which you have forcing concentrations rising, is more policy relevant".[105]
The TCR is much better known than ECS; there is a smaller intrinsic
uncertainty associated with it and, as the IPCC demonstrates,
a much better agreement between models and observational estimates.[106]
Although efforts to better understand and constrain ECS will continue,
Professor Allen told us that:
Everybody talks about it because everybody else
talks about it, but nobody can quite remember why we are talking
about it in the first place.[107]
45. In narrowing the possible range of estimates
for both ECS and TCR from observational measurements, the statistical
methodology of the IPCC was bought into question by Mr Lewis,
who explained to us that:
The bulk of the studies estimating [ECS and TCR]
use a subjective Bayesian method, not all of them. Mine uses an
objective method. One or two of them use non-Bayesian methods.
The non-Bayesian methods and the objective Bayesian methods give
very much the same answer. The subjective ones, basically the
answer you get depends on the subjective assumptions you have
fed in. [108]
From this Mr Lewis argued that the range of sensitivities
considered by the IPCC was too high. However, the IPCC gave consideration
to Mr Lewis' argument and concluded that:
Bayesian methods to constrain ECS or TCR are
sensitive to the assumed prior distribution. They can in principle
yield narrower estimates [
] but there is no consensus on
how this should be done robustly.[109]
The view of the IPCC was strongly reinforced by Professor
Allen, who also disagreed with Mr Lewis' analysis.[110]
The arguments concerning statistical methodology are likely to
continue both in the academic literature and future assessments
of climate sensitivity. For the time being, it is important to
realise that these arguments only affect one of a number of methodologies
for calculating sensitivity, all of which have been taken into
account by the IPCC to produce their likely ranges of ECS and
TCR.[111]
46. We also examined claims that uncertainties in
other areas of the IPCC report (such as those surrounding the
role of aerosols and natural variability) could lead to overestimation
of climate sensitivity.[112]
Professor Allen demonstrated that these uncertainties had been
accounted for by the IPCC through reference to a simple model.[113]
Mr Lewis disagreed and provided evidence to suggest that uncertainties
could lead to temperature projections that were below those of
the IPCC.[114] Professor
Allen responded:
These multimodel ranges are only considered "likely"
by IPCC, so the fact that it is possible to get below them by
assuming a maximally optimistic scenario should not be that surprising.[115]
A number of witnesses also pointed out that there
were unknown aspects of the climate that might lead to increasing
sensitivities as the planet warms.[116]
47. Some argued that, after the cut-off date for
submitting academic papers to be considered as part of the WGI
contribution to AR5, some results had been released that argued
for higher sensitivities, and some for lower.[117]
Professor Allen reflected on this and claimed that "there
is no clear pattern of results after the deadline pointing towards
lower or higher sensitivities than results published before the
deadline".[118]
The debates around climate sensitivity demonstrate the diligence
of the IPCC, as Professor Allen went on to illustrate:
I was part of the internal discussion group that
made the decision to lower the lower band on climate sensitivity,
which made a lot of noise at the time. I have had a lot of feedback
from colleagues in the scientific community about whether or not
that conclusion was justified. Papers have come out since the
IPCC report that point to substantially higher values for the
climate sensitivity and people invariably send me emails saying,
"See", when these papers come out. That is the process
at work. Scientists continue to check what other scientists have
done and obviously high profile statements, such as those made
in the IPCC summaries, are checked even more carefully than others.
They get a lot of scrutiny.[119]
48. The
WGI contribution to AR5 has considered the full range of both
Equilibrium Climate Sensitivity and Transient Climate Response
and given the best assessment possible within the constraints
of the evidence available at the time. It does not appear that
a consistent pattern for higher or lower sensitivities than that
stated in the WGI contribution to AR5 has emerged since its publication.
The "hiatus"
49. The observed global mean surface temperature
(GMST) reported by the IPCC for the last 15 years shows a much
smaller increase than over the past 30 to 60 years, despite the
decade of the 2000s being the warmest in the instrumental record.[120]
Depending on which data are used, the temperature rise from 1998-2012
is estimated to be around one-third to one-half of the trend from
1951-2012; this slowdown in warming has become known as the "hiatus"
or "pause".[121]
A number of respondents argued that the WGI contribution to AR5
did not adequately address this issue.[122]
For example, Professor Pierre Darriulat, former Research Director
of CERN and currently Professor of Physics at VATLY in Hanoi in
Vietnam, said that:
It is undeniable that the pause has come as a
surprise in a context where anthropogenic C02 emissions
keep increasing. It has obvious implications on factors that are
not properly taken into account in the climate models. As such,
it deserves a very critical study aiming at a proper evaluation
of the uncertainties attached to predictions. This is what should
be expected from a serious scientific approach.[123]
The WGI contribution to AR5 explicitly addressed
the issue of the hiatus. It concluded that it "is attributable
in roughly equal measure to a cooling contribution from internal
variability [such as changing patterns in the El Niño-Southern
Oscillation] and a reduced trend in external forcing [through
volcanic aerosols and a reduction in solar activity]".[124]
It also concluded that "barring a major volcanic eruption,
most 15-year GMST trends in the near-term future will be larger
than during 1998-2012".[125]
On this, an IPCC Review Editor commented:
The issues of a "lack of warming" over
the last 10-15 years was raised by several reviewers. I think
the way this has been handled via Box 9.2 [where the IPCC explicitly
addresses the hiatus] is outstanding. However, so far as I can
tell, Box 9.2 has not been subject to external review as a consequence
of timing. This is worrisome.[126]
50. Subsequent evidence has confirmed that a number
of witnesses supported the conclusions of the IPCC. For example,
Dr Stott told us that, "the assessment that was made in the
AR5 has been borne out by further evidence we have had since"
including the contribution from internal variability such as the
Pacific decadal oscillation which could play a role in the redistribution
of heat with in the oceans.[127]
Dr Stott's explanation of internal variability and increased ocean
heat uptake was re-iterated by several other witnesses.[128]
Additionally, we were referred, by a number of witnesses,
to the body of evidence released since the publication of the
WGI contribution to AR5 corroborating the IPCC conclusions on
the hiatus.[129]
Not all of the individual pieces of evidence published since
the WGI contribution to AR5 are consistent with the IPCC explanation
of the hiatus, but this is an area of on-going research that will
require long-term, robust data before firm conclusions on the
hiatus can be made.[130]
51. Regardless of the discussed explanations for
the hiatus, arguably the most frequent criticism of the IPCC was
its failure to predict the occurrence of the hiatus in any of
the earlier Assessment Reports. The IPCC acknowledged that only
3% out of 251 climate models managed to predict the hiatus as
defined above, although each Assessment Report since 1990 has
stated that warming will be non-linear and subject to influence
by natural variability.[131]
More recently, global climate models have factored in periods
of reduced warming, temperature stasis and cooling, though they
were not designed to be able to predict the timing of these events
precisely.[132] As
Professor MacKay, Chief Scientific Adviser to DECC, explained:
Many of the model runs do show slowdowns that
last as long as the latest slowdown. They happen at random times,
and it would have been an amazing coincidence if any of those
simulations had happened to precisely nail [this hiatus].[133]
We note that the hiatus contributed in part to the
IPCC decision to revise short-term model projections and slightly
lower the estimates for equilibrium climate sensitivity.[134]
According to Professor Lindzen, as the hiatus continues, it will
lead to further downward revisions of estimates of climate sensitivity.[135]
52. Amongst the debate surrounding the hiatus, one
point that was frequently re-emphasised is that the pause in GMST
must be considered in the context of the long-term global energy
budget (which includes all gains of incoming energy and all losses
of outgoing energy).[136]
In this regard, Professor Sir Mark Walport, Government Chief Scientific
Adviser, explained that:
The hiatus is the change in the atmospheric temperatures.
There is absolutely no hiatus in the totality of the global energy
inventory, which is going up relentlessly. [
] Well over
90% of the heat uptake is in the oceans, and that is continuing.[137]
This point reflects the fact that short term changes
in GMST are not good metrics for discussing overall warming,[138]
thus the WGI contribution to AR5 uses a variety of different measurements
to reach its conclusions.[139]
When asked whether Ministers had taken account of the hiatus the
Minister for Climate Change, Rt Hon Gregory Barker, told us;
There is a whole barrel of different evidence
that informs public debate and underpins our approach. As Minister,
I rely principally not on the story of the day but on the ongoing
scientific advice [
]. I think if you are making public policy
one has to rely on the qualified experts for the long-term advice
that they give you rather than be guided by whatever happens to
feature on a tabloid headline.[140]
53. Periods
of hiatus are consistent with earlier IPCC assessments that non-linear
warming of the climate is to be expected and that forced climate
changes always take place against a background of natural variability.
The current period of hiatus does not undermine the core conclusions
of the WGI contribution to AR5 when put in the context of the
overall, long-term global energy budget. Despite the hiatus, the
first decade of the 2000s was the warmest in the instrumental
record and overall warming is expected to continue in the coming
decades.
Climate models
54. The results of climate models dominate the second
half of the WGI contribution to AR5. They are used to simulate
a variety of near- and long-term global and regional climatic
behaviours. The results of the simulations are then used in the
projection of future climate changes and in the attribution of
the cause of observed climate changes.[141]
Climate models have continued to become increasingly sophisticated
and capable, now factoring in a greater level of complexity than
ever before.[142]
55. Critics have called into question a number of
issues surrounding climate models, particularly relating to short-term
projections for global mean surface temperature (GMST).[143]
Notably, less successful areas of the short-term regional model
outputs are used to bring into question the ability of the IPCC
to predict climate changes over the long-term and global scales.
For example, Professor Lindzen considered that:
You can add complexity to a model and that has
happened a lot, but it has not helped [the IPCC] do major things
with ocean processes that are still known to be important. We
like to think of progress as kind of linear. You spend more money,
you make it bigger and get it better, but there has been no discernible
increase in scale that has been associated with the advances.
There are claims that regional climate has improved, but there
are still notable errors in describing today's regional climate
with current models, so that naturally gives one pause with predictions
[
] If they can't get today's distribution of regional climate
right, why would they be reliable for the future?[144]
56. Under closer scrutiny, many of the criticisms
of model projections appear unfounded. Firstly, generalised statements
concerning their reliability do not reflect the complexity and
diversity of the models. As Professor MacKay illustrated:
There are many models. There are complex, general-circulation
models; there are simpler energy-balance models. One of the things
the IPCC did was to evaluate the models that are used, and this
evaluation looked at their abilities to simulate mean climate,
historical events in climate change, variability on multiple timescales
and regional modes of variability. There is a mixed scorecard.
There are some things that are modelled well but some things such
as regional variability are not yet well captured by the models.
They are improving in many of these areas. As the computers become
more powerful and the models are able to represent the earth's
system at higher resolution, the accuracy of many of these features
in the system is improving.[145]
This point was also emphasised by Professor Allen:
Yes, the IPCC does consider the reliability of
climate models and finds they are much more reliable for some
variables (like global temperature) than others (like local precipitation).
If the question is posed as "are the models reliable in all
respects" the answer is trivially "no".[146]
57. Secondly, many of the criticisms brought to our
attention gave a distorted impression of the models' performance
by selectively emphasising areas in which the models have behaved
less well (particularly across the hiatus period, see paragraph
49).[147] We note that
the WGI contribution to AR5 devoted an entire chapter to the evaluation
of climate models and demonstrated that:[148]
i) Models
generally have a good record of replicating GMST, and other climate
observations, dating back to the first IPCC report in 1990;[149]
ii) Models
have improved, and continue to improve, in terms of their agreement
both with themselves and with observed climate data;[150]
iii) The increasing
levels of complexity within models, though introducing additional
uncertainties, have helped to improve climate projections;[151]
iv) The models
are not perfect, their limitations are recognised by the IPCC
authors and poorer performing areas are highlighted and discussed
at length.[152] The
increasing use of performance metrics, which judge models against
their ability to reproduce observable climate data, has helped
to safeguard against undue weight being given to poorer quality
results.[153]
58. The process through which model results are interpreted
and used to generate projections for future climate shows an appropriate
level of critical scepticism by the IPCC authors. The output of
the climate simulations are subject to scrutiny and expert judgement
and the results are tested against the sensitivity towards their
initial conditions and key uncertainties.[154]
This fact was emphasised repeatedly, including by Professor Allen,
who said:
The key point is there is an enormous amount
of judgment in running climate models. There is nothing sacrosanct
about ranges that come out of the CMIP5[155]
ensemble. We rely on a whole range of lines of evidences, including
observations and these models, to provide these projections and
those are the projections that are elevated to the Summary
for Policymakers.
Just to pick up on the notion, "You no longer
rely on the models", there are certain things for which I
would never rely on these models and there are other things that
I would. That is where the expert judgment comes in. You have
to look at a model and ask yourself, "Is it appropriate to
the question that I am asking with it?" If you wanted me
to use one of these models to predict whether there is a white
Christmas in 2030, it wouldn't work, but that doesn't mean that
the model is useless for giving us the big picture estimates of
what warming we should expect over the coming century. There is
no mechanical process for taking a set of model results and turning
a handle and getting a set of answers out.[156]
59. It is widely acknowledged that decadal forecasts
and the difficulties and uncertainties associated with short-term
forecasting should not be confounded with the longer-term projections
and "broad direction of travel" in respect of overall
warming.[157] The Grantham
Institute for Climate Change at Imperial College London emphasised
the requirement to contrast the differences between the types
of projections:
We have less confidence in projected near-term
changes which are likely to remain heavily influenced by internal
variability [
] Nevertheless there is no evidence of a mechanism
that would significantly reduce the intensity of the projected
climate change by 2100.
If you do the assessment of the models against
a whole raft of climate indicators over those longer-term time
scales that you need to do to assess their fitness for purpose
for the longer-term changes [
] we see that the models do
a good job of representing the larger time-scale changes.[158]
It further criticised a pre-occupation with observations
and projections made on the decadal time-scale:
This emphasis on decadal temperature changes
is, we believe, ill-advised. In the context of climate change,
relatively short-term indices of the state of the climate system
have little relevance. It is quite conceivable for there to be
a future decade in which the Earth's surface temperature cools
even with continued climate change.[159]
60. We were pleased to see that officials at DECC
had a good understanding of both the strengths and weaknesses
of climate models. David Warrilow, Head of Climate Science and
International Evidence at DECC, highlighted that models are not
trying to project; they are trying to simulate a process:
When you come down to projection on the near
term, like one to two decades, we are into a very different set
of problems and I do not think there is an easy answer to doing
that. It is something that the scientific community is now working
on because they are conscious that there is an interest in trying
to project what would happen in the near term, but it is almost
analogous for weather forecasting. Weather forecasting is good
for up to four or five days, probably, but if you try to look
forward two to three weeks, the chaos of the system makes it much
more difficult to predict. Even predicting a season ahead is quite
difficult, but if you want to predict a decade you are trying
to predict a level of chaos that is very difficult to predict.
I think the IPCC has done the best that it can with the information
available, but I do not think people in the IPCC would say that
this is the final answer to the problem.[160]
61. Crucially, when considering climate change it
is important to recognise that the models represent one of multiple
lines of evidence. Professor Sir Mark Walport made it clear that:
It simply is not just models. There are an awful
lot of observations here as well. The models are part of the story
and the models look into the future, but they also use the past
to see whether they have modelled accurately. One of the interesting
things about the fifth report is how little changed it is from
the fourth report and the third report. What is happening is that
the uncertainty is gradually reducing. The very short report on
the physical science base is absolutely clear: warming of the
climate system is unequivocal. Each of the last three decades
has been successively warmer. The rate of sea-level rise has been
larger. Human influence on the climate system is clear. That is
the conclusion of the report. I think that is enough for policymakers
to start making decisions.[161]
62. Finally, we have found that the WGI contribution
to AR5 plays a pivotal role in encouraging the development of
more complex computer models, as pointed out by Professor Corinne
Le Quéré, Director of the Tyndall Centre for Climate
Change Research and an IPCC lead author, and her colleague Professor
Andrew Watkinson, Professor in Environmental Sciences, both based
at the University of East Anglia:
The IPCC process provides a major incentive to
co-ordinate modelling activities, without which it would be more
difficult to make intermodal comparisons and assess the reliability
of climate models [
]
One of the major advantages of the current system
is that it provides a focus for modelling activities in WGI, which
are essential to review the projections of climate change. Furthermore,
the assessment process provides the only forum where all the components
of the climate system can be looked at (e.g. the atmosphere, ocean,
ice, land, biogeochemical cycles), and consistency across different
streams of research can be analysed.[162]
It is essential that these vast, modelling infrastructures
are being used as effectively as possible and that consideration
is given to their future direction. Professor Allen questioned
whether modelling capacity was being used to answer the right
questions. He stressed the need to develop high resolution, regional,
real-time attribution systems:
The most obvious and important area in which
further effort is required to reduce uncertainty is the implications
of these global changes for regional climate and extreme weather
events. It is shameful, in my view, that we are still unable as
a community to quantify the role of human influence on high-impact
events like Typhoon Haiyan. The IPCC process may inadvertently
have contributed to this situation through its focus (mandated
by the commissioning governments) on long-term projections of
global climate change, which are necessarily reliant on coarse-resolution
models.[163]
63. A number of witnesses highlighted a requirement
to maintain and improve modelling hardware, for example the Met
Office Hadley Centre, a UK-based institute for climate modelling,
reported to us that "along with the need to run large ensemble
simulations, there is a clear need for supercomputing capacity
and infrastructure".[164]
In this regard, it is vital that modelling research institutes
receive the support they require if key uncertainties about the
climate are to be resolved. DECC responded to this point stating:
Just looking at the Hadley Centre, we have that
regularly reviewed and the last review indicated that we were
getting very good value for money from it; that the spend was
not dissimilar to what was being spent in other modelling centres
around the world and that it was at an appropriate level. The
only real question was whether we would be able to maintain our
position vis-à-vis computing power and that is something
that is now being actively pursued by BIS. I think we are looking
at the availability of computing power, which is one of the constraints
on getting better predictions.[165]
The Minister told us:
I think we definitely punch above our weight
in terms of the reputation and quality of the science that is
produced by those centres and it is a huge tribute to the people
that work there.[166]
64. The
models used in the IPCC's Assessment Reports have a successful
history of simulating past climate and their future projection
of substantial warming over the next century in all but the most
aggressive mitigation scenarios is well founded and overwhelmingly
clear.
65. In the light of the WGI contribution to AR5,
the Government should commission a strategic review of UK modelling
facilities to discern how current computing capacity could be
used more effectively to reduce remaining uncertainties. The review
should highlight areas of potential national and international
collaboration between modelling centres and any funding shortfalls
that need to be met.
71 Ian Strangeways (IPC 022), Dr Ruth Dixon (IPC 023),
Department of Energy and Climate Change (IPC 025), Met Office
(IPC 026), Royal Meteorological Society (IPC 029), Grantham Institute
for Climate Change (IPC 032), Royal Society (IPC 034), University
of Reading (IPC 035), Natural Environment Research Council (IPC 036),
Myles Allen (IPC 037), Corinne Le Quéré and Andrew
Watkinson (IPC 050), Grantham Research Institute on Climate Change
and the Environment (IPC 051) Back
72
Grantham Institute for Climate Change (IPC 032), University of
Reading (IPC 035), Corinne Le Quéré and Andrew Watkinson
(IPC 050) Back
73
Q1 [Professor Hoskins] Back
74
Q1 [Dr Stott] Back
75
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p869, The majority of the warming contribution from man-made
greenhouse gases is from carbon dioxide, but the IPCC also make
clear that other species (such as methane and nitrous oxide) are
of considerable significance. Back
76
IPCC Working Group I Contribution to AR4, Summary for Policymakers, Climate Change 2007: The Physical Science Basis
(2007), p10, A discussion of how the WGI contribution to AR5 treats
(un)certainty can be found on pages 138-142 of the IPCC Working
Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013). Back
77
Grantham Institute for Climate Change (IPC 032) Back
78
Roger A. Pielke Sr (IPC 011), Friends of Science Society (IPC 015),
Dr A Neil Hutton (IPC 028), Professor Richard Lindzen (IPC 047),
Judith Curry (IPC 052) Back
79
Professor Richard Lindzen (IPC 047) Back
80
Q92 [Professor Lindzen], Dr Norman Page (IPC 058), Dr A Neil
Hutton (IPC 028) Back
81
Q87 [Professor Lindzen], Q91 [Professor Lindzen], Christopher
Walter (IPC 005), Nongovernmental International Panel on Climate
Change (IPC 042) Back
82
Q9 [Dr Stott] Back
83
Q129 [Dr Shuckburgh] Back
84
The null hypothesis stipulates that the observed climate changes
would have occurred with no human intervention through internal
variability. Back
85
Q9 [Dr Stott] Back
86
Q27 [Dr Stott] Back
87
Q10 [Professor Hoskins] Back
88
Q92 [Professor Lindzen], IPCC Working Group I Contribution to
AR5, Climate Change 2013: The Physical Science Basis (2013), p886,
Box 10.2 Back
89
Q92 [Professor Lindzen], Richard Lindzen (IPC 068) Back
90
Q1 [Dr Stott], University of Reading (IPC 035) Back
91
Brigitte Nerlich (IPC 031), InterAcademy Council, Climate Change Assessments: Review of the Process and Procedures of the IPCC
(August 2010) Back
92
Q27 [Dr Stott] Back
93
Brigitte Nerlich (IPC 031), James Painter (IPC 044) Back
94
Brigitte Nerlich (IPC 031) Back
95
Q164 [Mr Painter] Back
96
Brigitte Nerlich (IPC 031) Back
97
Equilibrium Climate Sensitivity refers to the temperature change
to the surface of the Earth observed after a doubling of carbon
dioxide followed by enough time to allow all the different aspects
of the climate to reach equilibrium (a process that may take hundreds
of years). Back
98
Transient Climate Response refers to the warming to the surface
of the Earth observed after a doubling of carbon dioxide over
a 70 year period. Back
99
IPCC Working Group I Contribution to AR5, Summary for Policymakers, Climate Change 2013: The Physical Science Basis
(2013), p16 Back
100
As above Back
101
Q222 [Professor MacKay] Back
102
Q24 [Professor Hoskins] Back
103
Q24 [Professor Allen], Nicholas Lewis (IPC 017), Professor Richard
Lindzen (IPC 047) Back
104
Qq4-5 [Professor Allen], Q24 [Professor Allen], Myles Allen (IPC 078) Back
105
Q82 [Mr Lewis] Back
106
Q5 [Professor Allen] Back
107
Q5 [Professor Allen] Back
108
Q69 [Mr Lewis], Q83 [Mr Lewis], Nicholas Lewis (IPC 017), Nicholas
Lewis (IPC 070) Back
109
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p82-85 Back
110
Q24 [Professor Allen], Myles Allen (IPC 073), Myles Allen (IPC 078) Back
111
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p82-85 Back
112
Q68 [Mr Lewis], Q81 [Professor Lindzen, Mr Lewis], Professor Richard
Lindzen (IPC 047) Back
113
Q3 [Professor Allen], Myles Allen (IPC 073) Back
114
Nicholas Lewis (IPC 070) Back
115
Myles Allen (IPC 078) Back
116
Grantham Institute for Climate Change (IPC 032), Myles Allen (IPC 078) Back
117
Barry Brill (IPC 018), Myles Allen (IPC 037) Back
118
Q6 [Professor Allen] Back
119
Q4 [Professor Allen] Back
120
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p769 Back
121
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p192-194 Back
122
Christopher Walter (IPC 005), John McLean (IPC 016), Barry Brill
(IPC 018), Nongovernmental International Panel on Climate Change
(IPC 042), Professor Pierre Darriulat (IPC 049), Judith Curry
(IPC 052) Back
123
Professor Pierre Darriulat (IPC 049) Back
124
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p769-772 Back
125
As above Back
126
IPCC Working Group I Contribution to AR5, Reports of the Review Editors, Climate Change 2013: The Physical Science Basis
(2013) Back
127
Q47 [Dr Stott] Back
128
Q78 [Mr Lewis], Q198 [Professor MacKay], Q208 [Professor MacKay],
Department of Energy and Climate Change (IPC 025), Met Office
(IPC 026), Grantham Institute for Climate Change (IPC 032) Back
129
Met Office (IPC 026), Grantham Institute for Climate Change (IPC 032),
Natural Environment Research Council (IPC 036), Corinne Le Quéré
and Andrew Watkinson (IPC 050), Grantham Research Institute on
Climate Change and the Environment (IPC 051), WWF (IPC 054), Royal
Meteorological Society (IPC 074) Back
130
Q80 [Mr Lewis], Q208 [Professor MacKay], Friends of Science Society
(IPC 015), Royal Meteorological Society (IPC 029), University
of Reading (IPC 035), Natural Environment Research Council (IPC 036) Back
131
John McLean (IPC 016), IPCC Working Group I Contribution to AR5,
Summary for Policymakers, Climate Change 2013: The Physical Science Basis
(2013) p15, IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p769-772, Ed Hawkins, Tamsin Edwards and Doug McNeall,
Nature Climate Change, Vol. 4, March 2014, 154-156 Back
132
Q15 [Dr Stott], Q47 [Professor Hoskins, Dr Stott], Q198 [Professor
MacKay] Back
133
Q206 [Professor MacKay] Back
134
Myles Allen (IPC 037) Back
135
Q78 [Professor Lindzen] Back
136
Q118 [Dr Shuckburgh], Q123 [Dr Shuckburgh], Q198 [Professor Mackey,
Professor Walport], Q208 [Professor Walport], Q223 [Professor
Mackey, Professor Walport], Grantham Research Institute on Climate
Change and the Environment (IPC 051) Back
137
Q198 [Professor Walport] Back
138
Roger A. Pielke Sr (IPC 011) Back
139
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p38 Back
140
Q239 [Mr Barker] Back
141
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p867-1136 Back
142
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p741-866 Back
143
Christopher Walter (IPC 005), Roger A. Pielke Sr (IPC 011), Friends
of Science Society (IPC 015), Alan Gadian (IPC 020), Dr Norman
Page (IPC 058), Ian Strangeways (IPC 022), Dr A Neil Hutton
(IPC 028), Nongovernmental International Panel on Climate Change
(IPC 042), Professor Richard Lindzen (IPC 047) Back
144
Qq76-77 [Professor Lindzen] Back
145
Q194 [Professor Mackay] Back
146
Myles Allen (IPC 037) Back
147
Christopher Walter (IPC 005), Friends of Science Society (IPC 015),
John Christy (IPC 055) Back
148
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p741-866 Back
149
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p64-65 Back
150
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p824-825 Back
151
Q7 [Dr Stott], Met Office (IPC 026), IPCC Working Group I Contribution
to AR5, Climate Change 2013: The Physical Science Basis (2013),
p748-753 Back
152
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p815-816, p1013-1015 Back
153
IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p753-759, p809-810, p821-828 Back
154
Q12 [Professor Allen, Dr Stott], Q123 [Dr Shuckburgh], Q195 [Professor
MacKay], Q207 [Professor Mackay], Grantham Institute for Climate
Change (IPC 032), Myles Allen (IPC 073), Royal Meteorological
Society (IPC 074), Met Office (IPC 076), IPCC Working Group I
Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p959-961, p1009-1012 Back
155
CMIP5 stands for Coupled Model Intercomparison Project Phase 5
and represents a set of coordinated climate model experiments.
Back
156
Q13 [Professor Allen] Back
157
Q3 [Professor Allen, Professor Hoskins, Dr Stott],14 [Professor
Allen], Qq195-196 [Professor MacKay, Professor Walport], Q199
[Professor Walport], Met Office (IPC 026), IPCC Working Group
I Contribution to AR5, Climate Change 2013: The Physical Science Basis
(2013), p964-965 Back
158
Grantham Institute for Climate Change (IPC 032) Back
159
As above Back
160
Q237 [Mr Warrilow] Back
161
Q203 [Professor Walport] Back
162
Corinne Le Quéré and Andrew Watkinson (IPC 050),
University of Reading (IPC 035) Back
163
Myles Allen (IPC 037) Back
164
Q34 [Dr Stott], Met Office (IPC 026), Grantham Institute for Climate
Change (IPC 032) Back
165
Q253 [Mr Warrilow] Back
166
Q253 [Mr Barker] Back
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