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VIEWS AND OPINION

What would the world be like at 3°C of warming?

by Prof Nigel Arnell, University of Reading

n the Paris Agreement, countries committed to seek to limit the increase in temperature
to 1,5˚C above pre-industrial levels. However, even if countries fulfilled their current
Ipledges to reduce emissions, we would still see an increase of around 2,7˚C. 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˚C
or more.

So how different would the impacts of climate change be at 3˚C compared to 1,5˚C?
At the outset, it is important to point out that – even if impacts increased in line with
temperature – the impacts at 3˚C warming would be more than twice those at 1,5˚C.
This is because we already have an increase of around 1˚C above pre-industrial levels,
so impacts at 3˚C would be four times as great as at 1,5˚C (an increase from now of 2˚C
compared with 0.5˚C).
In practice, however, impacts do not necessarily increase linearly with temperature. In
some cases, the increase accelerates as temperature rises, so the impacts at 3˚C may be
much more than four times the impacts at 1,5˚C. At the most extreme, the climate system Prof Nigel Arnell
may pass some “tipping point” leading to a step change.
Two years ago, colleagues and I published research looking at the impacts of climate The real consequences for people
change at different levels of global temperature increase. We found that, for example, the will depend on how these direct physical
global average annual chance of having a major heatwave increases from around 5% over impacts – the droughts, the heatwaves,
the period 1981 to 2010 to around 30% at 1,5˚C but 80% at 3˚C. The average chance of a the rising seas – affect livelihoods, health
river flood currently expected in 2% of years increases to 2,4% at 1,5˚C, and doubles to 4% and interactions between elements of
at 3˚C. the economy.
At 1,5˚C, the proportion of time in drought nearly doubles, and at 3˚C it more than Our experience during Covid-19
triples (these are all global averages weighted by the distribution of population or cropland: tells us that what appear to be relatively
see the paper for details). modest initial perturbations to a system
Change in global average heatwave, river flood and drought risk with increase in global could lead to major and unanticipated
mean temperature: the individual lines represent different climate model projections of knock-on effects, and we can expect
regional change in climate, and the horizontal line shows the indicators over the period 1981 this with climate change too. If the
to 2010. Arnell et al., 2019, Author provided relationship between temperature
There is of course some uncertainty around these figures, as shown in the graphs increases and physical impacts like
above where the range of possible outcomes gets wider as temperature increases. There melting glaciers or extreme weather is
is also variability across the world, and this variability also increases as temperature rises, often non-linear, then the relationship
increasing geographical disparities in impact. River flood risk would increase particularly between temperature increases and
rapidly in south Asia, for example, and drought increases at faster than the global rate across the effects on people, societies and
much of Africa. economies is likely to be very highly non-
The difference between 1,5˚C and 3˚C can be stark even in places like the UK where the linear. All this means a 3˚C world will be a
impacts of climate change will be relatively less severe than elsewhere. In a recent study, lot worse than a 1,5˚C world.
colleagues and I found that in England the average annual likelihood of a heatwave as
defined by the Met Office increases from around 40% now to around 65% at 1,5˚C and over Nigel Arnell is Professor of Climate Change
90% at 3˚C, and at 3˚C the chance of experiencing at least one day in a year with high heat Science, and Director of the Walker
stress is greater than 50%. Institute, University of Reading, UK.
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 Acknowledgement
northwest of England from 10% each year now to 12% at 1,5˚C and 16% at 3˚C. As at the This article was first published by The
global scale, there is considerable variability in impact across the UK, with risks related to Conversation and is republished here in
high temperature extremes and drought increasing most in the south and east, and risks terms of the Creative Commons licence.
associated 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 Send your comments to
difference between impacts at different levels of warming is clear. rogerl@nowmedia.co.za

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