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IN the Paris Agreement, countries com- mitted to seek to limit the increase in temperature to 1.5℃ above pre-indus- trial levels. However, even if countries fulfilled their current pledges to reduce emissions, we would still see an increase of around 2.7℃. No wonder that nearly two thirds of Intergovernmental Panel on Climate Change (IPCC) authors who responded to a new survey conducted by the journal Nature expect the increase to be 3℃ or more. So how different would the impacts of climate change be at 3℃ compared to 1.5℃? At the outset, it is important to point out that – even if impacts increased in line with temperature – the impacts at 3℃ warming would be more than twice those at 1.5℃. This is because we al- ready have an increase of around 1℃ above pre-industrial levels, so impacts at 3℃ would be four times as great as at 1.5℃ (an increase from now of 2℃ compared with 0.5℃). In practice, however, impacts do not necessarily increase linearly with tem- perature. In some cases the increase accelerates as temperature rises, so the impacts at 3℃ may be much more than four times the impacts at 1.5℃. At the most extreme, the climate system may pass some "tipping point" leading to a step change. Two years ago colleagues and I pub- lished research looking at the impacts of climate change at different levels of global temperature increase. We found that, for example, the global average annual chance of having a ma- jor heatwave increases from around 5% over the period 1981-2010 to around 30% at 1.5℃ but 80% at 3℃. The aver- age chance of a river flood currently ex- pected in 2% of years increases to 2.4% at 1.5℃, and doubles to 4% at 3℃. At 1.5℃, the proportion of time in drought nearly doubles, and at 3℃ it more than triples. There is of course some uncertainty around these figures, as shown in the graphs above where the range of pos- sible outcomes gets wider as tempera- ture increases. There is also variability across the world, and this variability also increases as temperature rises, in- creasing geographical disparities in im- pact. River flood risk would increase particularly rapidly in south Asia, for example, and drought increases at fast- er than the global rate across much of Africa. The difference between 1.5℃ and 3℃ can be stark even in places like the UK where the impacts of climate change will be relatively less severe than else- where. In a recent study, colleagues and I found that in England the average an- nual likelihood of a heatwave as de- fined by the Met Office increases from around 40% now to around 65% at 1.5℃ and over 90% at 3℃, and at 3℃ the chance of experiencing at least one day in a year with high heat stress is greater than 50%. The average proportion of time in drought increases at a similar rate to the global average. The chances of what is currently considered a ten-year flood increases in the north west of England from 10% each year now to 12% at 1.5℃ and 16% at 3℃. As at the global scale, there is consider- able variability in impact across the UK, with risks related to high temperature extremes and drought increasing most in the south and east, and risks associat- ed with flooding increasing most in the north and west. Again, there is lots of uncertainty around some of these estimates, but the general direction of change and the dif- ference between impacts at different lev- els of warming is clear. The real consequences for people will depend on how these direct physical im- pacts – the droughts, the heatwaves, the rising seas – affect livelihoods, health and interactions between elements of the economy. Our experience during COVID-19 tells us that what appear to be relatively modest initial perturbations to a sys- tem can lead to major and unanticipat- ed knock-on effects, and we can expect this with climate change too. If the relationship between tempera- ture increases and physical impacts like melting glaciers or extreme weather is often non-linear, then the relationship between temperature increases and the effects on people, societies and econo- mies is likely to be very highly non-lin- ear. All this means a 3℃ world will be a lot worse than a 1.5℃ world. 6 maltatoday | SUNDAY • 7 NOVEMBER 2021 OPINION Nigel Arnell is Professor of Climate Change Science, Director of the Walker Institute, University of Reading COP26: what would the world be like at 3°C of warming? What appear to be relatively modest initial perturbations to a system can lead to major and unanticipated knock-on effects Nigel Arnell Rivers in Asia will become even more flood-prone