Claim: CO2 Causes Extreme Weather Even Without Global Warming

Guest essay by Eric Worrall

A new study by Professor Myles Allen, David Karoly and others claims that even if CO2 does not cause significant global warming, we still have to cap CO2 to prevent a “direct CO2 effect” from messing up the weather.

Rising CO2 may increase dangerous weather extremes, whatever happens to global temperatures

Press release issued: 11 June 2018

New research from the University of Oxford and collaborators at several other institutions, including the University of Bristol, provides compelling evidence that meeting the global warming target of 1.5°C may not be enough to limit the damage caused by extreme weather.

The paper, published today in Nature Climate Change, demonstrates that higher atmospheric CO2 concentrations directly increase temperature and rainfall extremes, meaning there could be dangerous changes in these extremes even if the global mean temperature rise remains within 1.5°C. The research highlights the need for climate policy to complement temperature goals with explicit limits on CO2 concentrations.

Much of the focus of climate change mitigation has been on the goal of limiting warming to 1.5°C warming agreed at the 2015 United Nations climate summit in Paris. However, the atmospheric CO2 concentrations required to limit warming to 1.5oC depend on the climate response. Researchers from Oxford and other institutions participating in the HAPPI-MIP project (Half a degree Additional warming, Prognosis and Projected Impacts Model Intercomparison Project) simulated future climate under the range of CO2 concentrations that all might be consistent with 1.5°C of global warming.

In the models, CO2 levels at the higher end of this range were shown to directly increase Northern Hemisphere summer temperature, heat stress, and tropical precipitation extremes. This means that even if a low temperature response helps us to meet the temperature target, there may still be ‘dangerous’ changes in extremes – in other words, severe weather impacts beyond those currently expected at 1.5°C.

The research points up the need to set explicit CO2 concentration goals to limit the adverse effects of high-impact weather extremes. It also supports existing findings that proposed geo-engineering solutions aimed at reducing global warming impacts without reducing CO2 concentrations may not be effective at counteracting changes in extremes.

Hugh Baker, DPhil student at Oxford’s Department of Physics and lead author of the research, said: “Future work is needed to confirm exactly why we see this direct CO2 effect, but current research points to a combination of circulation and cloud cover changes, and an increase in the amount of direct radiation on the Earth’s surface due to simply having more CO2 in the atmosphere.”

Oxford’s Professor Myles Allen adds: “This puts paid to the Pollyanna argument that we should wait and see before reducing emissions in case the global temperature response to rising CO2 turns out to be lower than current models predict. Hugh’s paper shows that the accumulation of CO2 in the atmosphere itself increases the risk of key damaging weather extremes, regardless of the global temperature response. It’s not enough to get lucky.”

Dr Dann Mitchell, a co-author of the paper from the University of Bristol’s School of Geographical Sciences, said: “Geo-engineering techniques that reduce the amount of sunlight hitting the Earth’s surface are increasingly thought of as a way of achieving the Paris Goals because they decrease surface temperature. However, our results show that for extreme climate such as heatwaves, changing the global mean temperature is not enough, you need to reduce CO2 concentrations themselves.”

The research was carried out in collaboration with researchers at the University of Melbourne, ETH Zurich, the University of Bristol and the National Institute for Environmental Studies in Tsukuba, Japan.

Source: http://www.bris.ac.uk/news/2018/june/rising-co2.html

The abstract of the study;

Higher CO2 concentrations increase extreme event risk in a 1.5 °C world

Hugh S. Baker, Richard J. Millar, David J. Karoly, Urs Beyerle, Benoit P. Guillod, Dann Mitchell, Hideo Shiogama, Sarah Sparrow, Tim Woollings & Myles R. Allen

The Paris Agreement aims to ‘pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels.’ However, it has been suggested that temperature targets alone are insufficient to limit the risks associated with anthropogenic emissions2,3. Here, using an ensemble of model simulations, we show that atmospheric CO2 increase—an even more predictable consequence of emissions than global temperature increase—has a significant direct impact on Northern Hemisphere summer temperature, heat stress, and tropical precipitation extremes. Hence in an iterative climate mitigation regime aiming solely for a specific temperature goal, an unexpectedly low climate response may have corresponding ‘dangerous’ changes in extreme events. The direct impact of higher CO2 concentrations on climate extremes therefore substantially reduces the upper bound of the carbon budget, and highlights the need to explicitly limit atmospheric CO2 concentration when formulating allowable emissions. Thus, complementing global mean temperature goals with explicit limits on atmospheric CO2 concentrations in future climate policy would limit the adverse effects of high-impact weather extremes.

Read more: https://www.nature.com/articles/s41558-018-0190-1

Sadly the full study is paywalled, but my guess is what we are seeing is a response to Lord Monckton’s proof that climate sensitivity to CO2 has been massively overcooked.

How do you perpetuate the terror, when you can no longer claim with a straight face that anthropogenic CO2 will cause substantial global warming? I guess we now have our answer – the solution is to claim that even if CO2 does not cause substantial warming, a “direct CO2 effect” will still mess up the weather.