A rise of a few tenths of a degree will have big consequences for the planet

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The Economist explains

THE PARIS agreement, negotiated at a United Nations summit in 2015, committed its 194 signatory countries (plus the European Union) to try and keep the world’s average temperature to “well below” 2°C above pre-industrial levels, and if possible 1.5°C. The world is already perilously close to that target. In recent years, the average global temperature has regularly been at least 1°C higher than those recorded at the end of the 19th century; in 2020, it was 1.2°C more. The World Meteorological Organisation, a UN agency, predicts that there is “at least a one in five chance of it temporarily exceeding 1.5°C by 2024”. Most of that warming has occurred since 1975, at a rate of about 0.15-0.2°C per decade. Accumulating greenhouse gases in the atmosphere make further warming inevitable. But what difference will a few tenths of a degree make?

1.5°C
The world is already experiencing increasingly erratic weather, including stronger storms and more unpredictable and extreme rainfall patterns, leading to deluges in some places and droughts in others. Even if global average temperature increases are stabilised at 1.5°C, that will still get worse. The annual probability of a heatwave—defined as four days with maximum temperatures above the 99th percentile of a normal warm season—in any given part of the world would rise from 5% to 28%. Sea levels would rise 40-80cm, swelled by melting glaciers, the breakdown of Arctic ice and the expanded volume of warmer oceans. That is more than enough to drown low-lying island nations such as the Maldives. Destructive wildfires like those seen in Australia and on America’s west coast in recent years will also become more frequent.

Not everywhere will be affected in the same way. Temperatures should rise fastest at the poles and high latitudes. The mercury should climb more slowly in the tropics, but they are more susceptible to severe changes in precipitation. Even these broad trends are uncertain—temperature rise is influenced by many factors, including landmass, and “hotspots” exist in various locations. (Parts of South Africa, for example, have warmed faster than northern Australia, despite being at similar latitudes. The Antarctic remains stubbornly cool compared to the Arctic.) And for many people, these changes have already happened. More than a fifth of the world’s population lives in areas where temperatures in the warmest seasons have already risen by more than 1.5°C (as an average between 2006 and 2015). Around one-tenth of the globe averaged temperatures 2°C or more above pre-industrial levels between 2014 and 2018, including the Arctic, parts of the Middle East, Europe and northern Asia.

2°C
According to the Intergovernmental Panel on Climate Change (IPCC), the body tasked by the United Nations with collating the science around global warming, there is a “robust difference” between the impact of 1.5°C of warming and that of 2°C. Sea levels could rise by an additional 10cm by 2100. Higher seas pose even greater threats to small island states as well as low-lying coastal and delta areas like Bangladesh and the lands around the Nile. Periods of extreme heat will also become even more likely. Cities such as Karachi in Pakistan or Kolkata in India could face annual heat waves like those in 2015, which killed thousands of people. More than 400m extra people could be exposed to extreme heat. Hundreds of millions more will face poverty as a result of a loss of agriculture and food insecurity. Many will be forced to migrate, alongside those displaced by rising oceans and possible increased fighting over scarce resources like food and water, says the IPCC. And the world’s economy will suffer as a whole: a 2°C rise could reduce global GDP per person by 13% by 2100 compared with no additional warming, according to a paper published by researchers at the University of Oxford in 2017. (A rise of 1.5°C would still reduce it by 8%.)

It is not just humans that will suffer. At 2°C of warming, 18% of insect species will lose more than half their habitats, as will 16% of plants and 8% of vertebrates. Almost all coral would be wiped out. (Even at 1.5°C, only 10-30% would remain.) These impacts on global biodiversity have consequences for human populations, too. Insects pollinate crops. Plants soak up carbon dioxide from the atmosphere. Coral reefs are nurseries for fish populations, thereby supporting human fisheries.

Beyond 2°C
The relationship between rising temperatures and damage is not linear. Three degrees of warming above pre-industrial temperatures would not be twice as bad as 1.5°C, or half-again as 2°C. The more temperatures rise, the more the world risks passing a series of tipping points, setting off changes that will fundamentally shift how the planet behaves (though scientists are uncertain when these will occur). One abrupt change would be the breakdown of the global ocean circulation system, which helps distribute heat around the world (eg, through the Gulf Stream) and determines regional weather patterns. Others include rainforests turning to savannah, collapses or shifts in monsoon patterns, the disintegration of Arctic or Antarctic ice sheets and vast amounts of methane being released by thawing permafrost. Each scenario could play out in ways almost impossible to predict. Consequences could range from the spread of deserts to the collapse of farmable land, drastic changes in weather patterns and further temperature rise. None is good.

The purpose of pledges to stabilise temperature rise at close to 1.5°C is to avoid the worst of these consequences. But despite the commitments made in the Paris agreement—and, in the years since, various countries’ plans to reduce their greenhouse-gas emissions to zero—the world is still on track to exceed 2°C of warming. (Even if all countries meet their targets, climate scientists predict that temperatures will rise by 2.3°-2.4°C above pre-industrial levels by the end of the century.) A seemingly small difference in degrees will have a big impact on how habitable the world remains.