Energy and Climate Change CommitteeWritten evidence submitted by the Met Office

1. The UK Approach to Carbon Budgets

The general approach to setting carbon budgets for the United Kingdom involves several stages. The first is to select a climate target based on an assessment of the consequences of different levels of future climate change. The Committee on Climate Change has focused on an approximately 50% chance of a global average near surface temperature increase of 2ºC above pre-industrial levels, and a much higher probability, >90%, of limiting warming to less than 4ºC above pre-industrial levels. Based on this climate constraint a set of global emissions pathways were then derived. These typically show a peak in CO2 equivalent emissions in the next decade followed by a long-term decline. The Committee then estimates an equitable share of emission reductions for the UK in the long-term and a transition pathway for the UK towards this long-term emission target.

2. Previous Contributions to the Work of the Committee on Climate Change

The Met Office Hadley Centre has provided evidence to inform various aspects of the Committee on Climate Change’s mitigation targets. Ahead of the first budget period it provided probabilistic simulations of global average near surface temperature change for a range of emissions scenarios provided by the Committee and by the Office of Climate Change.

The Committee on Climate Change’s 2016:4%low scenario corresponds to an equivalent CO2 global emission reduction of around 50% on 1990 levels by 2050. The Met Office Hadley Centre calculated this corresponds to a median warming of a little over 2ºC, with a probability of around 50% of exceeding 2ºC.

Additionally, several members of the AVOID consortium (http://www.avoid.uk.net/) provided advice to the Committee on Climate Change ahead of the fourth budget period on the science of large-scale climate system change and climate impacts. This concluded that whilst understanding of these issues had increased in several ways the evidence for setting a climate target based on the regional and system specific consequences of a 2ºC and a 4ºC global average rise above pre-industrial levels was still valid. However, a big regional spread in consequences was noted.

3. Met Office View on needs for Climate Science Updates

The original simulations performed by the Met Office for the first budget periods included uncertainties in the relationship between radiative forcing and temperature response, including the heat taken up by the ocean, along with uncertainty in the climate-carbon cycle feedback.

The science involved in modelling global emission scenarios of multiple gases to provide temperature projections is complex but new understanding of key processes and interactions mean climate models continue to become more physically realistic. For instance, a current version of the Met Office Hadley Centre Earth system model provides a three dimensional representation of the climate and carbon cycle, including simulating vegetation changes, atmospheric chemistry and a wider range of aerosol species, which are important for both climate change issues and air quality.

The latest simulations using this model reinforce the Met Office’s confidence in the earlier simple climate model simulations performed for the Committee on Climate Change. They indicate a reduction in emissions of the order of 50% on 1990 levels by 2050 corresponding to a global average warming of just over 2ºC during the 21st century, falling back below 2ºC by 2100.

Even with the improvements in modelling many uncertainties in future projected warming and climate change still remain. These include uncertainty in the precise magnitude of the response of clouds to a warming climate, uncertainty in the response of vegetation to CO2 increases in combination with climate change, and uncertainty on how climate change will alter the frequency and severity of extreme weather events. Some processes, such as loss of carbon from melting permafrost, is not yet included even in the latest complex climate models.

We recommend that the science of climate change and variability, and its impacts is periodically reviewed and at more frequent intervals then the usual IPCC timetable. Such a review should take account of the latest literature on the consequences of continued climate change, the relationship between emissions of climate forcing agents and the eventual climate response, and on additional earth system feedbacks as the science matures to a level at which they can be credibly included in the calculation of global carbon budgets.

We also note that current climate targets focus on average warming on the global scale. However, as the underpinning science develops, understanding grows and model resolution and complexity increase there is, and will be, increasing benefit from focusing at the regional level—particularly when considering changes in severity and incidence of extreme weather, and rapid changes in key systems such as the Arctic.

3.1 Specific issues of recent relevance

Climate sensitivity (sometimes referred to as equilibrium climate response) is relevant to studies that focus on very long-term change and, in particular, long-term stabilisation. The precise value of climate sensitivity is not known, but a probability distribution can be estimated using a variety of methods including: the recent energy balance of the planet, complex climate models, complex climate models constrained by observations, paleo observations. However, different methodologies lead to a range of probability distributions and it would be useful to take stock of the implications of these. It is fair to say that the most likely value has changed little over the last few years.

Transient climate response provides a better constraint on warming over the coming decades, and is particularly relevant to scenarios with increasing forcing. A number of recent estimates of the uncertainty in transient climate response have been made and these should be considered alongside the equilibrium climate sensitivity estimates above.

The effect of the slowdown in recent global average near surface temperature change has been noted by many commentators, although other aspects of climate continue to change in a manner consistent with our understanding of a warming world. Furthermore, there is evidence from both observations of the past and from climate models that temporary periods with less global average near surface warming than the long-term trend have happened before and are expected again in the future. Recent research has started to examine how the most recent period influences our view of parameters such as equilibrium climate sensitivity and transient climate response and it would be useful to consider implications of this for carbon budgets.

The interaction between climate change and the carbon cycle was factored into the first assessment of budgets. However, understanding of additional Earth system feedbacks, including from melting permafrost, was not sufficient to include in the first budget assessment. Whilst it is still limited there is a growing understanding that could now be considered. The Met Office Hadley Centre is already doing this in a number of projects. A recommendation is to consider the implications of the latest understanding on climate-carbon cycle interaction and additional earth system feedbacks.

Results from the AVOID programme have shown that whilst a 2°C global warming limit (with 50% probability) does appear still feasible, delays in global action make it more difficult to achieve. Whilst attention continues to focus on this target, the global role that might be played by permanent removal of CO2 from the atmosphere, referred to as negative emissions, merits further study.

This is particularly true when considering the impacts of geo-engineering options on a local or regional scale on the global and total Earth system. In addition, some geo-engineering options may prove to have irreversible effects in the long term and, even when considering short term options, may act only to mask, rather than reverse, the warming climate. For example, the use of aerosols in solar radiation management would act to cool the earth while CO2 is still being emitted.

Finally, the local consequences of climate change, which are likely to affect people, infrastructure and natural systems should be periodically reviewed. Newer models with higher spatial resolution and a greater vertical extent are providing improved skill at simulating many aspects of the dynamic behaviour of the atmosphere and oceans and offer advantages for looking at future climate variability and change. Additionally, the ability to simulate some impacts within climate models is opening up the prospect of improved assessments of quantities relevant to impacts.

4. Conclusion

There are some new developments in climate science that are appropriate to take into account when considering carbon budgets. Such developments are a normal part of the scientific progress.

As the science and understanding of the Earth’s physical, chemical and ecosystem processes continue to develop and mature, carbon and climate change policies generally should be flexible enough to accommodate breakthroughs in both modelling and in the underlying science. Consideration of how science developments in one area impact on another are also important.

5 June 2013

Prepared 3rd October 2013