Earth is warming. Globally, 2020 was the second-warmest year on record, with a mean temperature 1.2掳C above the pre-industrial average. By that measure, this means we are already four-fifths of the way to the 1.5掳C 鈥渟afe鈥� level to which the world committed to try to limit global warming.
The culprits are carbon dioxide emissions from burning fossil fuels and land use changes that reduce Earth鈥檚 ability to draw down greenhouse gases. The results are already being felt, not just through rising temperatures, but also through loss of ice cover, rising sea levels and more extreme droughts, floods and storms across the globe.
Greenhouse gas levels
In March 1958, climate scientist Charles David Keeling began measuring atmospheric carbon dioxide levels from a monitoring station atop Mauna Loa in Hawaii. The readings , now carried out by the US National Oceanic and Atmospheric Administration. Together with measurements of air trapped in ice cores collected from the Antarctic and elsewhere, our record of the concentrations of this crucial greenhouse gas stretches back 800,000 years.
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In March 2021, the average atmospheric CO2 concentration at Mauna Loa was 417.64 parts per million. Readings taken at other places around the globe confirm the picture at Mauna Loa: atmospheric CO2 concentration varies seasonally owing to differing levels of plant growth, but is trending upwards year on year. The average global concentration for 2020 was higher than at any point in the past 800,000 years.
The rise in CO2 we are seeing now began in the late 18th century, when the first industrialising countries in the West started mining and burning coal in large quantities. In the 20th century, ballooning population and consumption vastly increased the quantity of fossil fuels being extracted and burned. That, combined with humans claiming ever more land for crops and livestock, has resulted in skyrocketing levels of CO2 and methane, another potent greenhouse gas. A third greenhouse gas, nitrous oxide, released through agricultural activities, is also on the up.
Carbon emissions
Periods of economic contraction such as the 1973 oil crisis and the 2008 global financial crisis have produced brief downward blips in an otherwise relentless upwards trend of greenhouse gas emissions. The largest culprits are fossil fuels burned to produce electricity, propel vehicles, heat homes and power industrial processes such as the making of cement.
Early figures from the covid-19 pandemic suggest that it too will have no appreciable long-term effect on emissions. After an initial sharp fall in emissions during lockdowns in 2020, they have rebounded to nearly where they were before the crisis in lower and upper-middle income countries. They remain lower in high-income countries as economic activity continues to be depressed and many people stay at home.
Much of the recent rising emissions trend has been in emerging economies, China above all (see first graph). These countries are just playing catch-up, however: although the emissions of early industrialising economies in the West have flatlined or declined in recent years, they still dominate the list of highest per-capita carbon emitters.
Land use change is also a significant source of greenhouse gas emissions, as land is deforested and turned over to farming and other uses (see overleaf). Much of the excess CO2 ends up in the atmosphere, but the land and the oceans act as significant sinks, too.
Land and sea temperature
The result of excess carbon emissions is rising land and sea temperatures, as higher CO2 levels cause the ocean and atmosphere to take up and retain more heat.
On land, the five years from 2015 to 2019 and the 10 years from 2010 to 2019 are the hottest such periods on record. Since the 1980s, each successive decade has been warmer than any preceding decade since 1850.
More than 90 per cent of the excess heat is stored in the world鈥檚 oceans. Measurements at different depths show that surface waters have warmed first and fastest, but heat is now penetrating deeper. Ocean heat content was at a record level in 2020, with areas in temperate zones of the Atlantic and Pacific and towards the poles showing the biggest increases.
Ice cover and sea level
Increasing land and sea temperatures are beginning to have significant effects on Earth鈥檚 ice stores. Particularly hard hit is the Arctic, where the area of ocean covered by ice has seen a long-term decline throughout the era of satellite measurement from 1979 to the present day.
The trend is a loss of 540,000 square kilometres per decade, an area about the size of France. Antarctic sea ice grew a little over the same time period due to changing wind patterns, but this isn鈥檛 a sign that the Antarctic isn鈥檛 warming. Indications are that the Antarctic ice sheet is increasingly fragile.
The World Glacier Monitoring Service gathers data on glacier mass using a reference set of glaciers in 19 mountain regions. There have been 31 consecutive years of melting, with an average annual loss of 0.7 tonnes of water per square metre. The cumulative loss of ice since 1970 amounts to 21.1 tonnes of water per square metre.
As ice melts and oceans warm and expand, so sea levels rise. Sea level has risen steadily since January 1993, when the current high-precision measurements were first made. The average rate of rise is estimated to be around 3.2 millimetres a year over the 27-year-long period to 2020, but the rate has increased over that time.
Extreme weather
The effect of climate change on the frequency and intensity of extreme weather events is often hard to disentangle from natural variation. 鈥淐limate attribution鈥� is still an imperfect science, but modelling studies of the probability of individual events under different warming scenarios increasingly allow us to point the finger of blame.
The intensity of hurricanes and other tropical cyclones is increasing, and they tend to dump more water. Summer heat extremes, including in each of the past three years in Europe, have been conclusively shown to be exacerbated by climate change. Flooding events in Bangladesh, China, India, New Zealand, North America and southern Africa have also been made more likely.
Globally, wildfires may be becoming less frequent owing to changes in rainfall patterns, but are more intense and destructive when they do occur. In California, it is estimated that climate change contributed to an additional 4.2 million hectares of forest fire area between 1984 and 2015, almost double the area expected in its absence.
