What is global warming?

When you put on a coat, you warm up because you lose less heat. Similarly, as we put ever more carbon dioxide into the atmosphere, the entire planet is warming up because it is losing less heat.

The average temperature of the surface of the planet has already risen 1.1°C as a result of CO2 emissions from burning fossil fuels such as coal. The continents and the Arctic have warmed even more. The consequences are becoming ever more obvious, from record-smashing heatwaves and storms to mass die-offs of wildlife and coastal flooding as the seas rise.

And this is just the start. We are on course for at least a 3°C or 4°C rise in surface temperatures by 2100, and the planet could keep warming long after that. We are setting in motion vast changes that will take millennia to reverse if they can be reversed at all.

The entire Amazon rainforest could die off, for instance. Sea level is set to rise a metre or more by 2100, and could rise another 20 or even 40 metres over the following centuries as the ice sheets in Greenland and Antarctica are lost. Low-lying places like Florida are doomed.

While life has thrived during the many previous warm periods on Earth, this time is different. For starters, we humans have built cities and developed farming systems for climates that are fast vanishing. In the Arctic, for instance, buildings and roads are collapsing as the permafrost melts.

Secondly, natural causes of climate change such as alterations in Earth’s orbit produce gradual warming over thousands or millions of years. Our actions are making the planet warm at an unprecedented rate. Many plants and animals will not be able to migrate to cooler climates in time to survive – and unlike during previous warming episodes there are now roads and cities in their way. A sixth mass extinction is getting underway.

Thirdly, the climate isn’t going to stabilise for millennia. As we try to adapt to the new normal, it will keep changing. Venice has spent $6 billion on flood defences designed to cope with only with a 22-centimetre rise in sea level, for instance. They may not protect the city for long.

The threat is so serious that almost all countries around the world have signed up to the Paris Agreement, which calls for global warming to be limited to 1.5°C. But what countries have actually agreed to do would only limit warming to 3°C at best – and most countries are not on track to meet their pledges.

In fact, despite the rise in solar and wind energy, the world continues to burn ever more fossil fuels and pump ever more carbon dioxide in the atmosphere. So when climate activist Greta Thunberg says we are basically not doing anything about climate change, it’s a fair summary of the situation.

In the UK, the Committee on Climate Change have advised the government to set a binding target of reducing carbon emissions to net zero by 2050. This would mean making new cars electric-only by 2030, switching boilers and heating systems to green alternatives, and making huge and costly efficiency improvements to buildings. It would also involve drastic changes to the British landscape to facilitate tree-planting and carbon storage, and tighter regulations on industry emissions.

Some lines of evidence suggest that we are underestimating future warming, and that we could be heading for 5°C, 7°C or even more by 2100. That amount of warming would make some parts of the planet so hot it would be impossible to survive without air conditioning.

Finally, in case you’re wondering, because global warming leads to many climatic changes such as more intense rainfall, some people have long preferred the term climate change to global warming. In practice, both terms are used interchangeably.