Select Committee on Environment, Food and Rural Affairs Written Evidence

Memorandum submitted by the Met Office (U6)

  Note that the research at the Hadley Centre on which this submission is based, is largely funded by the Global Atmosphere Division of Defra, with additional resources from the Ministry of Defence and the European Commission. The scientific results have been published in the peer reviewed literature (eg Nature) or are being prepared for publication.


  1.  The terms of reference of this inquiry are concerned with the policies required to keep the UK on track in reducing greenhouse gas emissions and the government's role as chair of the G8 and as President of the EC in driving forward the Kyoto and post-Kyoto agendas. On the first point, the scientific work of the Hadley Centre underpins the UK government's commitment to reduce greenhouse gas emissions. This submission details some of the latest results, strengthening the evidence, on which the case for a reduction is based. On the second point, there are a number of key scientific results which highlight the regional and global consequences of a range of emissions scenarios which include a range of mitigation options. The main topics covered are:

    a.  The fingerprint of the effect of human activity on regional climate change.

    b.  The levels of greenhouse gases needed to stabilise climate.

    c.  High impact events under climate change.

    d.  Changes in extreme events in the next 100 years and measures of uncertainty. With a focus on major cities in G8 countries.

  2.  Key Hadley Centre findings on these topics are:

    a.  A considerable amount of evidence already suggests that much of the observed 20th century global warming has been driven by human activity. New evidence shows that human activity has caused warming on regional scales too.

    b.  Stabilisation even at quite modest levels of greenhouse gases may still require some adaptation, but early action slows the rate at which adaptation will be required. Preliminary calculations show that feedbacks between climate and the biosphere accelerate climate change, implying that lower emissions are needed to stabilise climate.

    c.  Even modest global warming in the next 100 years would lead to the melting of the Greenland ice sheet on much longer timescales (of the order of millennia). On the other hand, our results indicate that the Gulf Stream will probably weaken but not collapse.

    d.  Changes in extreme events are the way that most people will first experience climate change. Increases in summer extreme high temperatures are likely to be much larger than increases in mean temperature. For example in double CO2 conditions, mean summer temperatures are predicted to increase between 2-7ºC deg in London, Paris, Toronto and Washington, whereas extreme high temperatures are likely to increase by 10-15ºC. Also, the percentage of dry days is predicted to increase by more than 40% in many regions with associated decreases in summer mean rainfall of over 40%.

  We would be able to present some of these and other new results at the forthcoming G8 scientific conference at the Hadley Centre, recently announced by the Prime Minister.


Figure 1.  [Not printed, the information is available at]

  3.  Observations of temperature show that on average the globe has warmed substantially over the 20th century but that there have been large regional variations in the amount of warming. The Intergovernmental Panel on Climate Change (IPCC) concluded in their Third Assessment that there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities(1). Previous analyses have looked at temperature changes over the globe as a whole rather than over individual continents, but recently the Hadley Centre has examined the causes of 20th century temperature change on the continental scale. Here, the focus is on the landmasses of North America, Asia, South America, Africa, Australia and Europe. The modelling study investigated the historic impact on the climate system of:

    —  greenhouse gases alone;

    —  the combined effect of anthropogenic sulphate aerosol, lower atmosphere and stratospheric ozone; and

    —  the combined effect of volcanoes and changes in the output of the sun.

  The optimal detection method shows there is a significant greenhouse gas warming signal in all of the continental regions considered (Figure 1, left hand side). Temperature changes from other anthropogenic and from natural factors are detected in some but not all of the continental areas, since these forcings are weaker and more uncertain than greenhouse gas forcing. Therefore, we have more confidence in attributing a man-made greenhouse gas component to continental scale temperature changes than in attributing other factors. The right hand side of Figure 1 shows the temperature changes in the model results compared to the observations for each continental region. The increases in greenhouse gases caused increased warming as the century progressed. This was balanced to a greater or lesser degree, depending on region, by aerosol cooling. In general there is good agreement between observed and simulated changes.

  Footnote: (1) Page 158 of Climate Change 2001. Synthesis Report. IPCC.



  4.  The ultimate objective of the United Nations Framework Convention on Climate Change (UNFCCC) is to achieve ". . . stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous interference with the climate system." Even after greenhouse gas concentrations are stabilised, it will take a considerable amount of time for a balance to be reached between the incoming solar radiation and the heat lost from the planet. If greenhouse gas concentrations were stabilised today, which would require an immediate reduction in emissions of around 60%, we would still expect the temperature to eventually rise by another 1ºC due to the inertia of the climate system. Sea level rise may be more of a problem due to the long timescale for thermal expansion and potential melting of ice sheets.

  5.  We present our first estimates of the temperature and sea-level rise during the 22nd century for two scenarios:

    —  Proxy for stabilisation at 550 ppm—IPCC SRES B1 emissions and concentrations up to 2100 followed by constant greenhouse gas and aerosol concentrations at year 2100 levels for 100 years.

    —  Proxy for stabilisation at 750 ppm—IPCC SRES A1B emissions and concentrations up to 2100 followed by constant greenhouse gas and aerosol concentrations at year 2100 levels for 100 years.

  Note these scenarios imply unrealistically sharp reductions in emissions at 2100, but are used here to give a general impression of two approximate stabilisation concentration profiles. The following results were calculated using a simple climate model and will be repeated using our full coupled ocean-atmosphere model. Figure 2 shows that for both stabilisation scenarios the global mean temperatures continue to increase between 2100 and 2200, but at a reduced rate.

  6.  The sea level also continues to rise beyond the 2100 concentration stabilisation point. On this timescale the major component of the sea-level rise is the thermal expansion of the oceans. Because the oceans have a large thermal capacity they take a long time to adjust to changes in atmospheric greenhouse gas concentrations, and the commitment to future sea-level rise beyond 2100 (expressed as a percentage of the 21st century rise) is much greater than the commitment to temperature rise. The rate of sea-level rise 100 years after stabilisation is only slightly less than the rate when stabilisation occurred.

The effect of the carbon cycle on stabilisation of atmospheric CO2 concentrations

  7.  The concentration of CO2 in the atmosphere depends on the amount emitted—for instance, from the burning of fossil fuels and changes in land use—and the strength of carbon sinks, such as the ocean and biosphere, which remove CO2 from the atmosphere (Figure 3). As the atmospheric concentration of CO2 increases, so does the ability of vegetation to take up CO2 from the atmosphere (the carbon fertilisation effect). However, the increases in CO2 lead to changes in temperature and rainfall, which can affect natural carbon sinks. Over land, climate change can alter the geographical distribution of vegetation and hence its ability to store CO2. In the Hadley Centre coupled climate-carbon cycle model, we find that climate change results in a dying-back of the vegetation in northern South America. Climate change also affects the amount of CO2 emitted by bacteria in the soil. In the ocean, changes in circulation and mixing, which accompany climate change, alter the ocean's ability to take up CO2 from the atmosphere. In addition, the warmer oceans absorb less CO2. In order to include all of these feedbacks, it is necessary to treat the carbon cycle and vegetation as interactive elements in full global climate modelling (GCM) experiments. This approach was pioneered by the Hadley Centre and the first results were reported at CoP6 (Conference of the Parties, 6).

  8.  In principle, there are an infinite number of emissions pathways that lead to stabilisation of atmospheric CO2 concentrations at a given level. IPCC technical note 3 discussed two alternative emissions pathways which would stabilise CO2 concentrations at a particular level: the IPCC "S" emissions, and the emissions estimated by Wigley, Richels and Edmonds, the "WRE" emissions. These emissions were calculated using a simple carbon cycle model that took account of the carbon fertilisation effect, but other feedbacks—such as that associated with the change of vegetation patterns or the oceans, due to climate change—were not included. To investigate such effects we used a simple climate carbon-cycle model, which includes the feedbacks from vegetation, soils and the ocean. This reproduces the results of the full Hadley Centre coupled climate-carbon cycle model, and was used to make new estimates of the concentrations resulting from the WRE550 emissions scenario.

  Figure 4 above shows the CO2 concentrations that would result. Without including the feedbacks, the emissions eventually lead to stabilisation of CO2 concentration at around 550 ppm, as intended (below). However, when the more comprehensive feedbacks are taken into account, the CO2 concentration rises much higher—to 780 ppm by 2300. Thus, the effect of carboncycle feedbacks is to allow a greater fraction of CO2 emissions to remain in the atmosphere.

  9.  In summary, preliminary calculations show that including the feedbacks between climate change and the carbon cycle greatly reduces the "allowable" emissions that lead to CO2 concentration stabilisation at a given level. It does this by reducing the strength of carbon-dioxide sinks.


Will the Greenland ice-sheet melt?

  10.  The Greenland ice-sheet would melt faster in a warmer climate and is likely to be eliminated—except for residual glaciers in the mountains—if the annual average temperature in Greenland increases by more than about 3ºC. This could raise the global average sea-level by seven metres over a period of 1,000 years or more. We show here that concentrations of greenhouse gases will probably have reached levels before the year 2100 that are sufficient to raise the temperature past this warming threshold.

  11.  At present, about half of the snow falling on Greenland melts and runs off as water, and the remainder is discharged in the form of icebergs. Climate change caused by higher greenhouse-gas concentrations is expected to produce both higher temperatures and greater precipitation, but most studies conclude that the increase in melting will outweigh the increase in snowfall. For an annual average warming of more than 2.7ºC, the melting exceeds the snowfall—a situation in which the ice-sheet must contract, even if iceberg production is reduced to zero as it retreats from the coast. For a warming of 3ºC, the ice-sheet loses mass slowly and over millennia might approach a steady state in a smaller inland form. For greater warming, mass is lost faster and the ice-sheet is likely to melt away. Calculations of Greenland's temperature have been done using different scenarios of carbon dioxide increases followed by stabilisation over the next few centuries.

  Figure 5. Predicted warming of Greenland over the next few centuries. The dashed line is the 2.7ºC threshold. [Not printed, information is available at].

  12.  Figure 5 shows that the 2.7ºC threshold is passed in all but one of the 35 combinations of model and stabilisation level. The lowest carbon dioxide concentration considered was 450 ppm Given that this level is exceeded before 2050 in all of the IPCC report's emission scenarios, and that carbon dioxide is not the only greenhouse gas, we conclude that the Greenland ice-sheet is likely to be eliminated by anthropogenic climate change unless much more substantial emission reductions are made than those envisaged by the IPCC. This would mean a global average sea-level rise of 7 metres on millennial timescales.

  13.  This study has been extended at the Hadley Centre, by using its climate model to simulate and predict the evolution of the Greenland ice sheet over several thousand years. This experiment is novel in that changes in the ice sheet, such as the height of the ice or whether the ground is covered in reflective ice or dark soil, are fed back into the climate model.

  14.  Figure 6 shows that over the 3,000 years following a quadrupling of atmospheric greenhouse gas concentrations, the ice sheet recedes from most of Greenland. By the end of the simulation, it exists only on the mountainous ground of the East. The fresh water released from this loss would cause a sea level rise of around 7 metres. Earlier results suggested that if the ice sheet is removed in this way it would not recover, even if greenhouse gas concentrations were significantly lowered. The next task is to understand at what point the melt down of Greenland becomes irreversible.

  Figure 6. The Greenland ice sheet will melt over the course of 3,000 years. Red indicates thick ice while blue indicates thin (or no) ice. [Not printed, information is available at]

Will the Gulf Stream collapse?

  Figure 7. [Not printed, information is available at].

  Figure 7 Simulations using the HadCM3 climate model of the strength of the Atlantic thermohaline circulation from 1930 to 2000 (using historical variations of greenhouse gases, sulphate aerosol, solar radiation and volcanic dust). The simulations show a freshening of the Labrador Sea from 1950-2000, as has been seen in observations, but this is associated with a slight strengthening of the thermohaline circulation over the same period, rather than a weakening as has sometimes been suggested. When the simulations are extended forward from 2000 to 2080 (using a projection of future greenhouse gases and aerosols), both trends are reversed, with a salting in the Labrador Sea and a weakening thermohaline circulation.

  15.  A key question in climate research concerns the stability of the thermohaline circulation, a system of large scale currents including the Gulf Stream in the North Atlantic Ocean, which carries heat from the tropics to higher latitudes as cold salty water sinks near the poles, drawing warm water north-eastwards. Recent observations have shown a reduction in the amount of salt in the seawater deep in the northwest Atlantic, and this has been interpreted by some as an early sign of a weakening thermohaline circulation.

  16.  The Hadley Centre climate model (Figure 7) shows that the observations are in fact consistent with a slight strengthening of the thermohaline circulation since the 1960s. Nevertheless, the model predicts that in future it will weaken somewhat as a result of global warming. The model suggests a reduction in the strength of the Gulf Stream by as much as a quarter—but not a collapse. However, even with this reduction in the Gulf Stream, the net result of climate change will be a warmer Europe.


  17. Good progress is being made in predicting climate change on global and regional scales. Nevertheless, substantial uncertainties remain which are a hindrance to estimating impacts and hence to efficient adaptation. Uncertainty in emissions is dealt with by using a range of emissions scenarios (eg see above). The other major uncertainty is in modeling the climate system. To address this, the Hadley Centre has developed a suite of physically plausible models. So far an "ensemble" of 53 models (soon to be increased to over 100) has been used to estimate of the likelihood of different climate change events. We have extracted temperature and precipitation information from these results for major cities within the G8 countries.

  18. Changes in extreme events are the way that most people will first experience climate change. The Hadley Centre has focused initially on temperature and precipitation as there is most confidence in the results for these fields and they are of direct relevance to such concerns as heat stress, flooding, drought.

Summer temperature

  19.  Figure 8 shows changes in summer[2] temperatures from 53 different climate models arising from doubling atmospheric CO2 concentrations with respect to pre-industrial levels[3]. The blue bars represent the distribution of the changes in mean daily summer temperature by the different models. For example, Berlin has most models showing changes in the mean of between 2 to 5ºC with one model showing 11ºC. The red bars represent the change in the hottest[4] day in the summer with values for Berlin ranging from 3 to 19ºC but with the majority of models predicting changes of approximately 9ºC. This figure indicates that for most of the cities selected, changes in the extremes of summer temperature are likely to be far greater than the changes in average summer temperatures. Such changes in extremes will have significantly greater impacts than the more modest (although still substantial) changes in the means.

  Figure 8. Distribution of changes due to doubling atmospheric CO2 concentration in daily maximum temperature for the months June, July and August for 53 climate models. Grey bars represent mean daily temperatures and black "hottest day of season" (Average and "hottest day" defined as 50th and 99th percentile respectively).

Winter precipitation

  20.  To understand the changes in daily weather more fully it is useful to see how the whole distribution of possible daily values changes with increased CO2. A useful statistical measure to do this is percentile values. For example the 90th percentile is the value below which 90% of all the daily values reside. If we look at how increased CO2 changes each percentile we can form a picture of how the whole distribution is changing, both extremes and more normal values. Figure 9 shows this for changes in daily wintertime (December, January, February) precipitation due to doubling atmospheric concentration of CO2 for the same eight cities. The upper and lower lines represents the spread of results given by the 53 different climate models and so give a measure of the uncertainty in the future predictions. To illustrate, for London the range of changes in the 50th percentile span both negative but predominantly positive changes and with a central estimate of 10% increase in daily winter precipitation, so we would conclude that there is moderate confidence that typical wet days will be getting moderately wetter. For more extremes rainfall, as defined by the 80th percentile and above, all models show more substantial changes (>20%) with the uncertainty range significantly clear of 0% change, thus we would conclude that there is high confidence that extreme daily rainfall will increase substantially for London in winter.

  Figure 9. Changes in the distribution of daily wintertime (December, January, February) precipitation due to doubling atmospheric concentration of CO2. Presented as changes in all percentiles. Upper and lower lines represent the central 80% of results from 53 climate models.

Summer precipitation

  21.  For northern hemisphere summer much of the land surface shows an increase in the number of days without rain (Figure 10a). This is particularly so for European countries, Canada, central USA, Russia, Africa and the Pacific east coast. Substantial reductions in seasonal rainfall are also seen for many of these areas, with particularly severe drying over Europe and northern South America with seasonal reductions of more than 40% (Figure 10b).

  Figure 10(a) [Not printed, information is available at]. The average percentage change in days without rain in June, July and August for 53 climate models due to doubling atmospheric CO2 concentrations.

  Figure 10(b) [Not printed, information is available at]. The average change in mean June, July and August precipitation for 53 climate models due to doubling atmospheric CO2 concentrations.

Further work

  22.  In order to develop effective strategies for mitigation and adaptation the following areas need further work:

    1.  Better assessment of uncertainties.

    2.  More understanding of changes in extreme events (eg of temperature, precipitation).

    3.  More detail on regional scales.

    4.  Better understanding of the likelihood of high impact events.

    5.  A better link between work on predictions of climate change and the impacts of climate change.

    The Hadley Centre is involved in research on all of these issues.

28 September 2004

2   Summer and winter are defined here as the months June, July, August and December, January, February respectively. Back

3   All results presented here have given equal weighting to each of the 53 climate models. In reality some models will simulate current climate better than others and it would therefore be appropriate to give greater weight to the future predictions of such models. Although this will be incorporated in future analysis no such weighting has been applied here. Back

4   "hottest" in this context is defined as the 99th percentile. Back

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