While Covid-19 has shaken much of human society, the threat posed by global warming has not gone away.
Human activities have increased carbon dioxide emissions, driving up temperatures. Extreme weather and melting polar ice are among the possible effects.
What is climate change?
The Earth’s average temperature is about 15C but has been much higher and lower in the past.
There are natural fluctuations in the climate but scientists say temperatures are now rising faster than at many other times.
This is linked to the greenhouse effect, which describes how the Earth’s atmosphere traps some of the Sun’s energy.
Solar energy radiating back to space from the Earth’s surface is absorbed by greenhouse gases and re-emitted in all directions.
This heats both the lower atmosphere and the surface of the planet. Without this effect, the Earth would be about 30C colder and hostile to life.
Scientists believe we are adding to the natural greenhouse effect, with gases released from industry and agriculture trapping more energy and increasing the temperature.
This is known as climate change or global warming.
What are greenhouse gases?
The greenhouse gas with the greatest impact on warming is water vapour. But it remains in the atmosphere for only a few days.
Carbon dioxide (CO2), however, persists for much longer. It would take hundreds of years for a return to pre-industrial levels and only so much can be soaked up by natural reservoirs such as the oceans.
Most man-made emissions of CO2 come from burning fossil fuels. When carbon-absorbing forests are cut down and left to rot, or burned, that stored carbon is released, contributing to global warming.
Since the Industrial Revolution began in about 1750, CO2 levels have risen by around 50%. The concentration of CO2 in the atmosphere is higher than at any time in at least 800,000 years.
Other greenhouse gases such as methane and nitrous oxide are also released through human activities but they are less abundant than carbon dioxide.
What is the evidence for warming?
The world is about one degree Celsius warmer than before widespread industrialisation, according to the World Meteorological Organization (WMO).
Across the globe, the average sea level increased by 3.6mm per year between 2005 and 2015.
How will climate change affect us?
There is uncertainty about how great the impact of a changing climate will be.
It could cause fresh water shortages, dramatically alter our ability to produce food, and increase the number of deaths from floods, storms and heatwaves. This is because climate change is expected to increase the frequency of extreme weather events – though linking any single event to global warming is complicated.
As the world warms, more water evaporates, leading to more moisture in the air. This means many areas will experience more intense rainfall – and in some places snowfall. But the risk of drought in inland areas during hot summers will increase. More flooding is expected from storms and rising sea levels. But there are likely to be very strong regional variations in these patterns.
As more CO2 is released into the atmosphere, uptake of the gas by the oceans increases, causing the water to become more acidic. This could pose major problems for coral reefs.
Global warming will cause further changes that are likely to create further heating. This includes the release of large quantities of methane as permafrost – frozen soil found mainly at high latitudes – melts.
Responding to climate change will be one of the biggest challenges we face this century.