Estimating the risks of climate change: what are the effects of climate policy?

By Nigel Arnell

I am writing this from Beijing, where the 13th National People’s Congress has just reaffirmed the Chinese commitment to control future emissions of greenhouse gases and meet the aspirations of the Paris Agreement on Climate Change. This agreement, struck in 2015, commits countries to reduce emissions so that the increase in global mean temperature is limited to ‘well below’ 2 oC above pre-industrial levels, and to aim to limit the increase in temperature to 1.5 oC. I am here to attend the final workshop of a joint UK-China project which is providing policy makers in China and elsewhere with information on climate risks at the global and Chinese scales. We are looking at risks under ‘high’ emissions pathways, and are comparing these with the risks that arise under pathways that are consistent with the Paris Agreement. Policymakers want to know this partly so they can understand and communicate the risks of not reducing emissions, and partly in order to prepare for risks which cannot be avoided.

The analysis builds on several years of research funded by NERC and the Department for Business, Energy and Industrial Strategy (and its predecessors), most recently through the AVOID2 programme. The headline results from the AVOID2 programme were recently published by Lowe et al. (2017), showing that the impacts under high emissions were much larger than under lower emissions – particularly for impacts related to heat extremes – and also that in order to achieve the Paris targets it is necessary to reduce emissions rather sooner than implied in the commitments made by countries in Paris.

Figure 1 below (simplified from Arnell et al., 2018) shows the proportion of impacts that are avoided with a specific policy target (along the x axis), relative to the impacts that would occur at higher increases in temperature, for four different indicators of potential impact: the population exposed to drought, the area of cropland exposed to drought, the population exposed to flooding and the population exposed to heatwaves. The orange line shows the proportion of impacts that would be avoided with a particular policy target compared to impacts at an increase of 4 oC in 2100. Meeting the 1.5 oC target would reduce the population exposed to drought by around 80%, compared with a 4 oC world, and meeting a 2 oC target would reduce impacts by around 65%. The proportional effects are greater for the heatwave indicator and lower for the cropland drought indicator – suggesting that achieving climate policy targets would have the greatest effect on impacts generated by high temperatures.

Figure 1. The impacts avoided with different climate policy targets, relative to impacts at 2, 3 and 4 oC above pre-industrial levels. The plots show four impact indicators. The solid lines show the median estimate of the proportion of impact that is avoided, and the shaded area shows the uncertainty range.

The green line shows the proportion of impacts avoided relative to a temperature increase of 3 oC, which is close to the estimated increase that would occur if countries met the commitments they have already made to reduce emissions. Meeting the 1.5 oC target would reduce the population exposed to drought by approximately 65% and again the proportions vary between the indicators: heatwave impacts are reduced by around 90%. This suggests that additional efforts to reduce emissions beyond those already committed by countries would reduce future impacts substantially.

The blue line compares impacts at 1.5 oC and 2 oC, illustrating the benefits of moving to the more stringent Paris Agreement target. Drought impacts would be approximately 40% lower at 1.5 oC than at 2 oC, and heatwave impacts would be around 60% lower.

There are, of course, many caveats with this assessment. There is uncertainty in the projected benefits of climate policy, due to uncertainty in future regional changes in temperature and, particularly precipitation. This is shown by the shaded areas in the figure. There are also other potential indicators for the four impact areas shown here, and these may result in different apparent benefits of reducing climate change. The proportion of impacts avoided at lower temperatures varies between regions. However, together the results show that reducing emissions can reduce impacts substantially, and that impacts at 1.5 oC can be a lot lower than impacts at increases of 4, 3 or even 2 oC.

REFERENCES

Arnell, N.W., Lowe, J.A., Lloyd-Hughes, B. & Osborn, T.J., 2018. The impacts avoided with a 1.5 oC climate target: a global and regional assessment. Climatic Change 147: 61-76. Doi. 10.1007/s10584-017-2115-9. https://link.springer.com/article/10.1007%2Fs10584-017-2115-9

Lowe, J.A., Arnell, N.W., Warren, R., Gambhir, A., Bernie, D. & Thompson, E., 2017. Avoiding dangerous climate: results from the AVOID2 programme. Weather 72: 340-345. Doi 10.1002/wea.3176  https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/wea.3176

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