From wild weather to huge ecosystem shifts, dramatic effects from global warming are already baked in – but by acting now we can prevent the worst.
IT IS probably the best deal we can hope for – but it is considerably worse than the one we have now. “It is not going to be fine,” says Dann Mitchell. “We will see significant changes to our lives.”
Mitchell, a climate modeller at the University of Bristol, UK, is talking about a world warmed by 1.5°C – the number agreed at the Paris climate talks in 2015 as the “safe” level of global warming we should aspire to stay below.
The political will to keep to the 1.5°C target has been lacking so far. The latest round of talks, which closed in Katowice, Poland, last month, confirmed the number, but gave no indication of how to stick to it. Meanwhile, the number’s scientific significance has grown, as both the minimum warming we can possibly achieve, and the maximum that we can tolerate without near-certain disaster.
But what does a 1.5°C warmer world really look like? And if we miss the target and end up at 2, 3 or 4°C, what then? The answers are beginning to crystallise. They aren’t pretty – but they do tell us what’s at stake.
When world leaders in Paris unexpectedly announced their intent not just to keep warming below 2°C compared with pre-industrial levels, but also “to pursue efforts to limit the temperature increase to 1.5°C”, delegates cheered. Although the actual targets set in Paris still commit the world to more than 2°C of warming, the adoption of 1.5°C has really focused the minds of climate scientists, says Myles Allen at the University of Oxford, a leading light at the Intergovernmental Panel on Climate Change (IPCC).
Our current best estimate is that the world has already heated by 1°C, with the temperature increasing at 0.2°C per decade. There are still significant uncertainties on this number, however – and a tenth of a degree either way really matters if 1.5°C is the new goal. “When we were talking about 2 or 3 or 4°C of warming we didn’t care about this,” says Allen. “But now we’re at 1°C of warming and worried about stabilising at 1.5, it is really important.”
Such uncertainties cloud our forecast. “Research into the impacts of 2°C has increased in recent years, as well as studies into 4°C and beyond, but there has been very little attention to 1.5°C,” says Mitchell. The IPCC’s report “Global Warming of 1.5°C“, issued last October, sets out what we do know.
Much depends on how we achieve 1.5°C. Most feasible pathways now require an overshoot, in which we temporarily go over the threshold before clawing our way back. The larger and longer this overshoot is, the greater the dangers are, and the more difficult the forecast becomes.
To provide a definitive view of the future, you need to model a world that has stabilised and equilibrated at 1.5°C, says Mitchell. That is the aim of his recently launched project Half a degree Additional warming, Prognosis and Projected Impacts, or HAPPI. His team is also modelling 3 and 4°C of stabilised warming, which with gallows humour they call the UN-HAPPI project. But even before they are done, we can make some general predictions about a world that’s 1.5°C warmer. The difference might not sound like much: if the temperature rose or fell by that amount on a mild day, you would hardly feel it. But on a global scale it will be very noticeable.
Today’s 1°C warming is uneven, with some places already 1.5°C warmer or more. Unfortunately for terrestrial species such as humans, land areas are likely to bear the brunt of future warming. The IPCC report warns that “for almost all areas of human habitation, temperature increases will exceed, frequently by more than one-and-a-half times, the global average”.
That makes a difference. At mid-latitudes – everywhere except the polar regions and tropics – the hottest summer days will be some 3°C hotter than before 1900. Heatwaves, defined as three or more consecutive days among the 10 per cent hottest days in the 31 days around it, will also become more common, especially in the tropics.
In a 1.5°C warmer world, 15 per cent of Earth’s population will endure a heatwave classified as “severe” once every five years. Even worse, “extreme” heatwaves such as the one that hit western Europe in 2003 will affect one in 10 people once every 20 years. That heatwave caused between 35,000 and 70,000 deaths.
Right now, about one in 10 summers in London are cool enough that the heat-related death toll is zero. With 1.5°C warming, that will go down to one in 25. On the plus side, fewer cold days will mean fewer cold-related deaths. But that gain is likely to be outweighed by more deaths related to extreme heat.
Closer to the equator, the effects will be more severe. At 1.5°C, twice as many megacities with a population of 10 million or more, mainly in tropical regions, are forecast to become heat-stressed, exposing more than 350 million more people to potentially deadly heat (see “Unequal burdens”). Droughts driven by heat extremes will also become more common, but exactly where and to what extent is hard to say. The Mediterranean, for one, appears to be getting more drought-prone. “An important finding is the projected expansion of deserts in the Mediterranean to a level unparalleled in the past 10,000 years if temperatures rise above 1.5°C,” says Rachel Warren of the Tyndall Centre for Climate Change Research in the UK. Each month an average of 100 million more people are expected to be exposed to drought, with attendant risks such as an increased incidence of wildfires.
But too much rain will end up being a bigger problem in the 1.5°C warmer world. Warmer air holds more moisture, and rainfall will become heavier and more common overall. The hardest-hit regions will be those that are already wet, and often populous – places such as northern Europe, eastern North America and east Asia. Alaska, Canada, Greenland and Siberia will also get a lot more precipitation.
That sinking feeling
Extreme precipitation events such as tropical cyclones – the class of storm that includes hurricanes and typhoons – aren’t expected to become more frequent, but they will become more intense and wetter. “They will kill a hell of a lot of people and cause financial damage and generally wreak havoc,” says Mitchell. Around three-quarters of the world’s population will face twice the rainfall-related flood risk they do today, with the largest increases in risk in the US, Asia and Europe.
Water from the sky is one menace, but water from the sea is another. Depending on when we get there, 1.5°C warming is projected to cause between 26 and 77 centimetres of sea level rise relative to 2005, says Warren. That puts at least 136 port megacities at risk of inundation. By 2100, up to 150 million people will be directly affected by sea level rise, through effects including flooding, erosion, contamination of drinking water, inundation of agriculture, decline of fisheries and loss of ecosystem services such as storm defences from mangroves and other coastal ecosystems. Ice takes time to melt, so even if the climate has stabilised at 1.5°C warmer in 2100, sea levels will continue to rise for decades or even centuries to come, says Warren.
“Too much, not too little, rain will be the problem in a warmer world”
Meanwhile, the oceans themselves will be changing. Over the past 35 years, the number of marine heatwave days has doubled. With half a degree more warming, this tally is expected to increase a further eightfold. Species of fish, marine mammal and plankton will be able to adapt to higher temperatures by relocating. But kelp forests and coral reefs are stuck and will experience high rates of mortality and loss. “At 1.5°C, already a 70 to 90 per cent decline in existing coral reefs is projected,” says Warren.
Coastal ecosystems such as seagrass, salt marshes and mangroves will also suffer. Kelp, coral and seagrass are “ecosystem engineers”, building physical structures that create habitats for other life – coral reefs support over a million species, for example – and also provide food and protection to people.
Acidification caused by dissolved carbon dioxide will also affect a wide range of organisms, from fish to algae. The oceans absorb about 30 per cent of our emissions and their pH has fallen by 0.1 units since pre-industrial times – a change unprecedented in the past 65 million years. One model projects that catches of marine fish will decrease by 1.5 million tonnes a year under 1.5°C warming.
Land ecosystems will see significant changes, too, says Warren. Up to 10 per cent of the world’s land surface could transform from one ecosystem to another. Forest ecosystems will be affected by drought, intense storms, wildfires and pest outbreaks. Around 20 per cent of tropical forest is likely to become savannah or grassland. This could potentially lead to major deforestation at a time when forests are badly needed to soak up CO2. Meanwhile, woody shrubs will encroach northwards into the tundra as up to 7.5 million square kilometres of permafrost melts – more than 40 per cent of what was there in 1960. Melting permafrost releases the greenhouse gases CO2 and methane, further exacerbating warming.
These ecosystem changes spell trouble for terrestrial wildlife. Climate change will directly shrink the geographical ranges of many species, on average by about 25 per cent. Many species will lose more than half of their range: 10 per cent of amphibians, 8 per cent of reptiles, 6 per cent of mammals, 5 per cent of birds, 10 per cent of insects and 8 per cent of plants. Wildlife will also be lost to fires, extreme weather and the spread of invasive species, pests and diseases. Some species may be driven to extinction, although we don’t yet have an estimate of how many.
These changes at sea and on land will have a significant impact on food security: warming is expected to reduce the productivity of livestock farming, fisheries and aquaculture. Despite hopes that increased warmth and CO2 could make terrestrial crop agriculture more productive, real-world yields, particularly in the tropics, appear to be falling as the climate warms. Faster growth under conditions of higher CO2 will also lead to lower levels of some nutrients in crops.
An extra 95,000 children a year are projected to die of malnutrition in a warmer world, and 48,000 more from diarrhoea, according to the World Health Organization. Mosquitoes carrying malaria, dengue fever, chikungunya, yellow fever and Zika virus will spread, with 60,000 extra deaths expected from malaria. Europe and North America can expect to see more cases of West Nile virus and Lyme disease.
All these developments spell economic trouble. The IPCC calls climate change a “poverty multiplier”, meaning that more people get dragged into poverty and people who are already poor get even poorer. All told, global warming of 1.5°C could plunge more than 100 million people into extreme poverty. Migration and conflict are expected to increase as a result, though making precise forecasts of this is all but impossible. And it isn’t just developing economies that will suffer: each degree of warming, for example, is expected to damage the US economy to the tune of 1.2 per cent of GDP per year.
It all sounds pretty bad. But it could be a lot worse. For one thing, on the way to 1.5°C warmer it seems unlikely that we will meet any climate tipping points at which the climate system suddenly lurches into some new state. A classic example is if the permafrost were to melt completely, dumping billions of tonnes of greenhouse gases into the atmosphere.
“We don’t have much science to suggest that at these levels of warming we will see some critical change,” says Mitchell. By the same token, a cautious person would say we don’t have much science to suggest that we won’t. “Tipping points are something we find really difficult to model,” says Mitchell. “We have a high degree of uncertainty about them for the simple reason that many of them we have never observed.”
That is the danger posed by overshooting: the higher and longer we overshoot, the more likely it is that we will encounter a tipping point. “If warming were to exceed 1.5°C before returning to that level by 2100, there would be a greater level of risk,” says Warren. “Some of the impacts incurred may be irreversible, including loss of some ecosystems.” The current trajectory of greenhouse emissions threatens warming of 4°C by 2100. Commitments made in Paris will get us to 2.9°C at best.
What does this mean for our chances? Mitchell reckons that if we throw enough money at adaptation, warming of 1.5°C is manageable. “It is going to get a lot hotter, but we can just spend more money on air conditioning, for example.” But every bit of extra warming makes it harder to adapt. Everything that happens at 1.5°C warmer still happens, but more frequently and extremely. Corals, for example, will die off completely. Beyond 2°C, even the rich world probably can’t cope.
If that doesn’t focus minds on shooting for just a 1.5°C rise, perhaps nothing will. “There are clear benefits to limiting global warming to 1.5°C compared to 2°C, or higher,” says Warren. “Every bit of warming matters.”
This is the third in a series of New Scientist features looking at the effort to limit global warming to 1.5°C agreed at the Paris climate talks in 2015
First published at the New Scientist – 16 January 2019. See; https://www.newscientist.com/article/mg24132130-500-earth-in-2050-this-is-what-a-world-warmed-by-1-5c-looks-like/