Expert comment: Global temperatures might not respond to mitigation efforts for decades08 July 2020
New research published in Nature Communications, and led by the Center for International Climate Research in Norway, shows that it could take decades for reductions in human emissions to lead to detectable changes in global surface temperatures.
Prof Richard Allan, Professor of Climate Science, University of Reading, said:
“It is well known that inertia in the climate system will mean that the rapid reductions in greenhouse gas emissions required to mitigate dangerous climate change will take many decades to produce noticeable effects compared with the decade to decade natural fluctuations in climate. For example, the recent, temporary emissions reductions due to COVID lockdowns will not perceptibly reduce warming of climate. This study provides an elegant and systematic assessment of how long it takes for sustained emissions reductions to produce a noticeable effect on mitigating climate change, showing that CO2 and methane are the most important agents as expected.
“To detect a noticeable reduction in global warming they concentrate somewhat arbitrarily on a mid-range ‘what if’ emissions scenario. Scientists don’t know exactly how much warming each emissions pathway will produce, since clouds amplify climate change to varying degrees in different simulations. In reality, we will only have the past temperature record as a guide in gauging whether political action is producing a discernible effect on climate. What this and many other studies underline however is that it is only with rapid, substantial and sustained emissions reductions can we avoid the most dangerous climate change that is expected based on all the scientific evidence.”
Prof William Collins, Professor of Meteorology, University of Reading, said:
“This study used the innovative step of combining a simple climate model (which can distinguish small changes in temperature) with the variability (“noise”) from complex climate models. This has allowed the authors to address many more scenarios. The results do rely on the simple climate model being able to simulate the effects of different climate pollutants, so some caution is needed (as the authors acknowledge) on the predicted effects of aerosol reductions.
“Our understanding of past climate change and how it might evolve in the future is based on robust understanding of the underlying physics. Samset and co-authors show it will take until the 2040s to see the benefits in terms of reduced warming resulting from even strong decreases in our emissions of carbon dioxide. This means that assessing our progress towards meeting the temperature goals of the Paris Climate Agreement will need to rely on our physical understanding rather than measurements of climate change. By the time we have observed a reduction (or lack of reduction) in warming it will be too late to take further action.
“Reducing pollutants that last for only a short time in the atmosphere (short-lived climate pollutants) such as methane or soot have been suggested as a way of seeing a quick climate benefit. This study confirms that this would indeed be the case for a drastic cut in these pollutants, but for a more realistic gradual reduction the effects are seen no more rapidly than for carbon dioxide.
“Climate policies to combine reductions in carbon dioxide and short-lived climate pollutants are most likely to show both the earliest evidence of the results of our actions, and to lead to the lowest warming in the long term. Even so, for the next couple of decades or more it will be important to manage expectations of seeing the benefit of the wide social and economic transformations that will be needed to avoid the most damaging impacts of climate change.”