July 7, 2020

expert reaction to study looking at when global temperatures might respond to mitigation efforts

A study, published in Nature Communications, looked at when global temperatures might respond to mitigation efforts.

Prof Grant Allen, Professor of Atmospheric Physics, University of Manchester, said:

“This exciting study provides new insight into the impact of emissions mitigation strategies.  It highlights the long-understood lag and inertia in the way the climate system responds to changes in the concentrations of greenhouse gases in the atmosphere.  As the authors state, the study does not provide explicit quantitative predictions, but rather gives an impression of the timescales of detectable change related to emissions scenarios.  It adds to and reinforces something we have long known about Earth’s climate system, which is that there is a lag and inertia in terms of the ultimate climate effect of any emissions today, which can take many decades to reach equilibrium.  This is because greenhouse gases exist in the atmosphere for many decades or centuries once emitted, and because of complex natural feedbacks in the Earth system and the role of the ocean as a massive heat energy reservoir.  Work already underway will need to continue to refine our understanding of these processes.

“The study reinforces our understanding that climate change is a long term problem that will not simply disappear if all human-related emissions stopped tomorrow.  We will have to live with climate consequences that have been locked in already in decades past, and are exacerbated every day that we emit more greenhouse gases than the Earth system can remove.  It will take longer still to undo the changes that we have locked in over recent decades.  The summary is that we have already stored up problems for our future.  But this is no reason not to limit the damage now. R educing emissions now will help to limit the damaging changes we will continue to see in the future.  But we do need to realise that adapting to already locked-in climate change is something that cannot be avoided as this study demonstrates.  There is no quick fix.”

Dr Friederike Otto, Acting Director, Environmental Change Institute, and Associate Professor in the Climate Research Programme, University of Oxford, said:

“Of course we known that global mean temperature doesn’t immediately respond to changes in greenhouse gas emissions and we have painfully experienced how difficult this fact makes the public discourse during the so-called hiatus earlier this century where greenhouse gas emissions increased but global mean temperatures didn’t increase quite as much as in the years before and in the most recent decade.  Therefore I think this study is important, as it helps to do expectation management and gives as an idea about the possible timeframes in which the global temperature will respond to emission reductions.  It is also really important to highlight that this is an exploration of possible timescales and not a prediction.

“The study uses one of the largest ensembles of global climate models there is and so one of the best tools currently available to do this kind of study.  Of course it is one model only and so it is an exploration of the possible response times as simulated in this model, and not a prediction or a projection but it also never pretends to be anything else and to do expectation management that is exactly what we need.”

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

“The work is quite idealised as it looks at the mitigation of individual pollutants and reduces them by fixed annual amounts.  Results are also focussed on comparing to one future emission pathway.  This isn’t the emission pathway we are currently on, and in reality mitigation measures reduce many pollutants simultaneously.

“By focussing on 10 year trends the authors present an unnecessarily gloomy view of how much influence we can have on our climate in the next decades.  Our own work finds that society can have a discernible cooling effect on the Earth’s temperature over the next 15-20 years with serious mitigation efforts.”

Dr Amanda Maycock, Associate Professor in Climate Dynamics, University of Leeds, said:

“The study’s authors acknowledge they explore idealised emissions reductions that differ from the path we need to follow to achieve the targets of the Paris Agreement.  Although, as this study shows, natural fluctuations in climate can obscure the effects of climate trends on a 10 year timeframe, our ongoing work suggests that making rapid and ambitious reductions in multiple emission sources gives us a very good chance of avoiding extreme warming rates in the next two decades.”

Prof Tim Palmer, Royal Society Research Professor, University of Oxford, said:

“It bothers me a little that some non-experts may interpret the word “delayed” in the title of this paper as meaning “compared with what we previously thought”.  Fundamentally these are not new results: just as we knew that it takes some time for the climate-change signal to emerge from the noise on the way up, we knew that it will take some time for a reduced climate-change signal to emerge from the noise on the way down.  However, the authors have done an extensive investigation of this effect and thus we have a better quantification of the effect than we previously had.”

Prof Gabi Hegerl FRS, Professor of Climate System Science, University of Edinburgh, said:

“The paper shows that natural climate variability combined with the inertia of the system means that it will take a while to see a discernible difference in how the actual climate evolves compared to projections without mitigation, or with less mitigation.  This is a point made before, but this paper now shows how different greenhouse gases and forcings can contribute to a discernibly reduced warming rate.  It shows that for a substantial difference in warming rates that persists, sharply reducing carbon dioxide emissions is key, while reducing methane and black carbon helps also.  I hope we keep this in mind as we restart the economy after COVID.  To avoid reaching high and dangerous levels of global warming with all its impacts, we need to rapidly reduce carbon emissions.

“The study is statistically sound and carefully considers natural variability by using data from a large ensemble.  Of course, the underlying model for the response to forcing is fairly simple which may affect the details of the response.”

Prof Andrew Watson FRS, Royal Society Research Professor, University of Exeter, said:

“This is a solid study, using reduced complexity emulator modelling.  The results are consistent with what we know about the time scales on which the climate system responds.

“This study illustrates a major problem that we have with tackling climate change.  The climate system is so massive that altering it is like turning a supertanker.  We have spent many decades steering it in the wrong direction, and it will take decades for the results of climate mitigation to be obvious.  Society and politics normally operate on much shorter time scales and this big, lasting problem is a challenge to deal with.”

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

“It is well knows 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.”

‘Delayed emergence of a global temperature response after emission mitigation’ by B. H. Samset et al. was published in Nature Communications at 16:00 UK time on Tuesday 7 July 2020.

DOI: 10.1038/s41467-020-17001-1

Declared interests

Prof Grant Allen: “I hold NERC funding to research the quantification of global greenhouse gas emissions.  I have no link to this study or its authors.”

Prof Piers Forster: “No competing interests.”

Prof Tim Palmer: “No interests to declare.”

Prof Andrew Watson: “I’m a Royal Society Research Professor based at the University of Exeter, I am not involved in the research in this paper.”

Prof William Collins: “No conflict of interest.”

None others received.