New research indicates that the latest observations of the climate system’s response to rising greenhouse gas levels are consistent with conventional estimates despite a “warming pause” over the past decade. However, the most extreme rates of warming predicted by the current generation of climate models over 50- to 100-year timescales are looking less likely.
The findings, resulting from a broad international collaboration of scientists, have just been published in Nature Geoscience. Researchers used the most up-to-date information on temperatures, energy flows and energy accumulation in the climate system, to re-assess climate models.
Our colleagues at the UK SMC collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476; smc@sciencemediacentre.co.nz).
Dr Richard Allan, Reader in Climate Science at the University of Reading, said:
“This work has used observations to estimate Earth’s current heating rate and demonstrate that simulations of climate change far in the future seem to be pretty accurate. However, the research also indicates that a minority of simulations may be responding more rapidly towards this overall warming than the observations indicate.”
“Sunlight reflected back to space by aerosol pollutant particles, which offsets some of the heating from greenhouse gases, is difficult to measure, as is the heating rate of the deep ocean. Both make it difficult to estimate the most realistic rate of future global warming, but they don’t change the overall picture and certainly don’t give us cause for complacency.”
“It is important to understand how much the planet will warm up in response to radiative forcing from rising greenhouse gas concentrations. This is often quantified as the total warming experienced in response to a doubling of the carbon dioxide concentration: how sensitive climate is to this heating effect (climate sensitivity).
“However, since the total response of the climate system can take hundreds of years to reach its final resting place (or equilibrium), more useful for making policy decisions involving adaptation strategies is the journey to this final resting place, or how quickly the climate responds (transient climate response).
“Are climate simulations, used to project future changes in climate, realistic in both of these respects? To answer this question, Otto et al combine knowledge of the extra energy entering the climate system due to rising greenhouse gas concentrations and other factors (radiative forcing) with observations of surface temperature and of how heat is building up (primarily within the oceans).
“Despite the slow rate of surface warming in the recent decade, energy has continued to build up within the oceans and the authors find that the inferred sensitivity of climate to a doubling of carbon dioxide concentrations based on these observations (1.2-3.9 C total warming) is more or less consistent with the range from climate simulations (2.2-4.7 C). However, the observations suggest that the rate of warming up to the point of doubled carbon dioxide concentrations over the coming decades may be slightly lower than predicted by a few of the climate models used to make future projections.
“The authors caution that uncertainties in the observations and the cooling effects of aerosol pollutant particles mean that it is difficult to precisely anticipate the most realistic rate of climate response over the coming decades, but with work like this our predictions become ever better.”
From the AusSMC:
Professor Steven Sherwood, co-Director of the Climate Change Research Centre at the University of New South Wales, comments:
“These authors have looked at recent warming and ocean heat content data, and found that the oceans are sequestering heat more rapidly than expected over the last decade. By assuming that this behaviour will continue, they calculate that the climate will warm about 20% more slowly than previously expected, although over the long term it may be just as bad, since eventually the ocean will stop taking up heat. However, there is other research pointing out that this recent storage may be part of a natural cycle that will eventually reverse, either due to El-Nino or the so-called Atlantic Multidecadal Oscillation, and therefore may not imply what the authors are suggesting. So while their conclusions are interesting, they need to be taken with a large grain of salt until we see what happens to the oceans over the coming years.”
Dr Steven Phipps is a Research Fellow in the ARC Centre of Excellence for Climate System Science at the University of New South Wales, comments:
“This new study refines our estimates of the Earth’s sensitivity to increasing concentrations of atmospheric carbon dioxide. Using the latest observations of temperature and heat uptake, the authors calculate the short-term and long-term warming that follows a doubling of the CO2 concentration. Their “best estimate” of the short-term warming is slightly lower than previous results. The most likely short-term response to a doubling of CO2 is a global warming of 1.3ºC, while the most likely long-term response is a warming of 2.0ºC. Once uncertainty is taken into account, these new estimates are consistent with previous work.
“However, by including observations from the past decade, this study provides the most accurate estimates yet of the climate sensitivity. The extra precision confirms what we have long known: that our planet faces a very uncomfortable future if our emissions of greenhouse gases continue unabated.”