December 6, 2017

expert reaction to research suggesting greater future global warming

The global warming projection for the end of the twenty-first century could be about 15 per cent greater than the steepest emissions scenario from the Intergovernmental Panel on Climate Change (IPCC), reports a study published in Nature.

Prof. Mark Maslin, Professor of Climatology at UCL, said:

“Our climate models are running too cold, in fact 15% too cold according to climatologists Brown and Caldeira in the journal Nature, making the Paris climate agreement 2˚C target even more ambitious than we thought. Climate models are essential for us to understand the consequences of emitting greenhouse gases. The models are built from our fundamental knowledge of physics and how the climate works. But for any given future emissions scenario the models give a large range of potential increased global temperatures.

“This new study has ground truthed the models with the very latest measured satellite climate data, in particularly the measure of the Earth’s energy budget. This has allowed them to develop robust statistical methods to correct the climate models to what has been observed therefore narrowing the range of predicted future global temperatures.

“The additional strength of this study is that instead of using a single climate model which can vary greatly in their sensitivity of greenhouse gas emissions, they have used all the models in the latest phase of the Coupled Modelled Intercomparison Project – which produces the modelled output for the IPCC. Hence they have checked and corrected all the models used to predict future climate change. This has revealed that the models are underestimating the potential warming by up to 15% – which means international action to keep global temperature below 2˚C or even 1.5˚C will mean cutting carbon emissions deeper and faster than previously thought. To achieve these targets the climate negotiations must ensure that the global emissions cuts start as planned in 2020 and continue every single year thereafter.”

Prof. William Collins, Professor of Meteorology at the University of Reading, said:

“This study is a step-change advance in determining how much future warming we can expect. There have been many previous studies trying to compare climate models with measurements of past surface temperature, but these have not proved very conclusive in reducing the uncertainty in the range of future temperature projections. The Brown and Caldeira study instead breaks the issue down into the fundamental building blocks of climate change which are the changes in energy flows (fluxes of short-wave and thermal infrared radiation). These energy flows have now been measured for long enough by the CERES satellite instruments that they can be used in conjunction with the climate models to give a more tightly constrained projection of future warming.

“This study finds that the models that are slightly more sensitive to carbon dioxide than average agree better with these new measurements. In particular the energy fluxes seen by the satellites suggest that the amount of sunlight reflected back out of the atmosphere by clouds and the surface will decrease in a warmer world thus amplifying the effects of man-made climate change. So we are now more certain about the future climate, but the bad news is that it will be warmer than we thought.”

Prof. Richard Allan, Professor of Climate Science at the University of Reading, said:

“Climate models represent the most complete and comprehensive physics of Earth’s climate system.

Combined with observations, they become laboratories to advance understanding of this complex system and are scientist’s only tools for conducting realistic projections of how climate will evolve as atmospheric greenhouse gas concentrations continue to increase in response to human activities

“In their important study, Brown and Caldeira use satellite observations of sunlight reflected by the planet and infra-red energy radiated out to space to improve certainty in projections of the future. They achieve this by linking present day climatology and variability with future changes and find that the amount of sunlight reflected by clouds is important.

“They further discover that climate change may be larger than assumed when considering only the raw climate model projections, to the tune of half a degree Celsius of extra warming when comparing 2100 to today, assuming that the deep cuts in greenhouse gas emissions required to avoid dangerous climate change are not achieved.

“The analysis focuses on global average temperature projections and does not account for all the unknowns yet highlights the importance of combining simulations with observations of Earth’s energy balance, critical in determining the driving force for and climate response to rising greenhouse gas concentrations.

“These results add more evidence that model simulations with more realistic representation of today’s climate predict the greatest warming over the coming decades in response to atmospheric pollution from human activities.”

Prof. Piers Forster, Professor of Climate Change at the University of Leeds, said:

“We’ve known for a while that climate simulations with the best clouds generally predict more future warming than the rest. This new study puts the earlier finding on a much firmer statistical footing, suggesting the future temperature change will be 15% greater than our current best estimate would suggest.

“We need to be careful though, this is only one line of evidence. Other lines of evidence based on the historically observed warming suggest the simulations with slightly cooler projections may fit best. We need to consider all the lines of evidence before we jump to conclusions.”

Prof. Dave Reay, Chair in Carbon Management at the University of Edinburgh, said:

“As each year passes, our climate future in the 21st century becomes clearer. This update of projections using observed data helps to reduce uncertainty and to inform climate models. However the biggest unknown in all this is just how fast and far-reaching our efforts to tackle climate change will be. The Paris Climate Agreement, and aggressive mitigation by communities, cities and states around the world, can ensure the catastrophic warming pathways in climate models forever remain theoretical.”

* ‘Greater future global warming inferred from Earth’s recent energy budget’ by Patrick Brown & Ken Caldeira published in Nature on Wednesday 6 December 2017.

Declared interests

None received.