Parliament has given the CCC the responsibility to constantly monitor the science upon which we depend to guide our responses to the threat of climate change. Like all scientists, we always take a sceptical stance, testing each assertion against the evidence and ensuring proper peer review of every important finding. Although we turn to the experts to find the best available facts, we assess material from other sources carefully, to ensure that we have missed no useful insights.
The Mail on Sunday recently published an article by David Rose challenging fundamental aspects of climate science and suggesting that action now to reduce greenhouse gas emissions is unwarranted. In accordance with our responsibility, we considered the three main claims, which focus on:
- Agreement between observed temperatures and climate model predictions
- The recent pause in global temperature rise
- ‘Climate sensitivity’ (defined as the temperature change after a doubling of carbon dioxide concentration in the atmosphere).
We conclude that the approach to global and UK emissions reductions underpinning the Climate Change Act remains appropriate despite the assertions in the article. Nonetheless, as ever, it will continue to be important to monitor developments in climate science closely and draw out any policy implications.
Of the four scientists quoted in the Mail article, three (Myles Allen, James Annan and Piers Forster) have since publicly criticised it. The chart used appears to have been copied from the blog of a fifth climate scientist, Ed Hawkins, who has also taken issue with the article’s interpretation of it.
A chart of observed global temperatures against climate model outputs is the main evidence provided in the article. It claims that the chart “blows apart the scientific basis” for reducing emissions. This is simply incorrect, and reveals a misunderstanding of what the chart shows – a pattern of observed temperature over the last sixty years within the range of model outputs (see detailed notes below).
Even during the last 15 years of negligible temperature increase, observations have remained within the predicted range. At the same time, other climate indicators (e.g. ocean heat, sea level, sea ice cover and mountain glaciers) demonstrate that the Earth system as a whole is continuing to heat up. Analysis suggests that the underlying temperature trend is still rising once the known, shorter-term factors such as El Niño, volcanic aerosols and solar variability are filtered out. Other model evidence suggests that the excess heat is being exported to the deep ocean during this period of stable temperature at the surface.
The Mail article moves from the lack of recent temperature increase to arguing that climate sensitivity has been significantly overstated. But while it is true that recent studies based on temperature observations question the very highest model projections of warming, even if confirmed, these would not justify a wholesale downward revision of the range as argued in the article. Other lines of evidence continue to support the high end of the range for climate sensitivity (see detailed notes below).
The scientific basis for significant long-term climate risks remains robust, despite the points raised by the Mail. Early and deep cuts in emissions are still required, motivating the UK’s 2050 target in the Climate Change Act, the carbon budgets towards this target, and the measures to meet carbon budgets such as investment in low-carbon power generation technologies.
The results from recent studies are evidence of a healthy climate science community continually testing its science. In addition to the issues discussed here a number of uncertainties remain, for example as regards the carbon cycle, cloud effects, regional climate, extreme weather change, and impacts. And there is an extensive work programme underway to try to resolve these uncertainties. Given what we know now, the appropriate response to these uncertainties and associated risks is to cut emissions rather than to wait and see. We will continue to monitor closely developments in climate science and draw out any implications for policy.
Professor Sir Brian Hoskins is a member of the CCC, Fellow of the Royal Society, Director of the Grantham Institute for Climate Change at Imperial College and Professor of Meteorology at the University of Reading.
Dr Steve Smith is climate science advisor on the secretariat of the CCC and has a PhD in atmospheric physics.
More about how we arrived at these conclusions can be found in the detailed note below.
Individual climate model runs show global temperature fluctuating year-on-year around longer-term trends. However, they are not designed to predict the exact short-term fluctuations in actual temperature due to weather around the world. Different model runs produce different sequences of fluctuations. These are plotted together to give a range within which the real temperature should fall, provided the models are accurate and enough runs have been done to sample the full range of fluctuations.
The chart in the Mail article shows the central range in which 50% of the model results are found and an outer range in which 90% are found (the article seems to have mislabelled these ranges). Hence 5% of the model runs are below the bottom of the 90% range and 5% above the top of the range. With accurate models we would expect about 3 years of observed temperature in the past 60 to be below the 90% range and 3 years above it.
The observed temperature behaviour is consistent with model outputs, even during the last 15 years of negligible temperature increase, when temperatures have remained within the 90% range. Similar periods of limited or negative temperature change during greenhouse gas warming are reproduced in individual climate model runs – the fact that temperature can remain flat for several years is to be expected, and is not evidence against longer-term warming due to greenhouse gases.
The Intergovernmental Panel on Climate Change (IPCC) estimated in its last assessment (AR4, published in 2007) that climate sensitivity likely lies in the range 2-4.5°C.
There are various lines of research which inform this range:
- Climate models, which aim to simulate all the relevant processes in the atmosphere, land and ocean. In these models climate sensitivity is an emergent property which can be measured as an output, rather than an a priori input.
- ‘Observationally-based’ studies. These combine estimates of observed global temperature, radiative forcing and global heat uptake in a simple energy balance equation to derive climate sensitivity.
- Estimates based on past climate variations, either large changes such as the ice ages, or smaller recent changes in response to volcanic eruptions.
Several studies along all these lines have been published since AR4. When the latest temperature observations are used to calibrate the climate models, one study found that the top of the range of warming projections for the next few decades looks unlikely, but the bulk of the range remains likely. Other work indicates that aerosol pollutants which cool the climate may be offsetting greenhouse gas warming less than previously thought. Since this slightly reduces the amount of underlying warming that can be attributed to greenhouse gases, it too suggests the most extreme future projections are less likely, while still keeping most of them in play.
Other studies continue to support the upper end of the climate sensitivity range. One study which calibrated models using atmospheric humidity rather than surface temperature found that the more sensitive models are more realistic. A large research project to estimate climate sensitivity from the various natural climate changes over the last 65 million years found a range of 2.2-4.8°C. Therefore there are uncertainties to be resolved, but the evidence does not suggest the need to halve the entire IPCC range as argued in the Mail article.
Our advice is framed around an overall objective to keep a 50:50 probability of a temperature rise close to 2°C and a negligible chance of reaching 4°C by 2100. Modelling work with the Met Office Hadley Centre in 2008 showed that global emissions therefore ought to peak by 2020 and halve by 2050. Results showed that this still gave a 10% chance of reaching at least 3.5°C. If new findings confirm a reduction in this high-range possibility this would clearly be good, but would not suggest that delaying action is a sensible way to meet our objective.
The next IPCC report covering warming projections is due to be published this Autumn. We will consider any implications in our review of the Fourth Carbon Budget at the end of the year.