It has been getting a little warm lately. In November 2015, Brits experienced the hottest November days ever recorded in the UK. That was shortly followed by news from the World Meteorological Organization that 2015 is likely to be the hottest year since records began.
Global temperatures are now set to reach 1 °C above pre-industrial levels. That is halfway to the politically-agreed upper limit of 2 °C, which was set by world leaders in 2009.
For now at least, it seems the world’s temperature is going to go up and up. So how hot could the Earth really get? Is there any limit to the amount of warming humans could eventually cause?
Climate change is not a new experience for Earth. The planet has gone through countless temperature fluctuations over its 4.6-billion-year history, from frozen snowball to blazing tropical heat.
Without the greenhouse effect, the Earth would have an average temperature of -18 °C
But despite all these changes, Earth always swings back into roughly the same temperature range. That is because it has mechanisms in place to control its own temperature.
A key one is the greenhouse effect. Greenhouse gases in the air, mainly carbon dioxide, methane and water vapour, trap radiation from the Sun and act like a thermal blanket around the planet.
Without the greenhouse effect, the Earth would have an average temperature of -18 °C and be covered in ice. Life as we know it would not be able to survive. The greenhouse effect is clearly a good thing, but like all good things, it is possible to have too much.
Humans have only been here for a relatively short time, yet we have managed to become the most significant driver of climate change on the planet. By burning fossil fuels and cutting down trees, we are releasing more and more carbon dioxide into the atmosphere, and that has caused temperatures to rise.
By the end of this century the world will be at least 4 °C warmer than it was before the Industrial Revolution
Between 2000 and 2010, our rates of emissions more than quadrupled from the previous decade, and so far there is little sign of this upwards trend reversing. The question is, how much will all these extra greenhouse gases heat the planet over the coming decades and centuries?
To predict the state of the planet in the future, scientists build computer models that simulate what will happen to the Earth’s climate. These models are vastly complicated, but they ultimately rely on basic physics such as how air and water behave. By incorporating both man-made and natural changes, the models can estimate how the climate will change when a given amount of greenhouse gases is emitted.
These predictions are compiled into reports by the Intergovernmental Panel on Climate Change (IPCC), most recently in 2013-2014. They suggest that, if our greenhouse gas emissions continue to increase as they have been over the last 50 years, then by the end of this century the world will be at least 4 °C warmer than it was before the Industrial Revolution.
What’s more, the warming will not stop at the end of this century.
The further forward we go in time, the harder it is to predict what will happen.
Models currently suggest that we will reach 7 °C above pre-industrial levels by 2200, but that temperatures will then stabilise, provided we have stopped emitting greenhouse gases.
Climate models can only make predictions based on what we know
However, we cannot be certain of this, because the Earth’s climate is a complex system. As the climate warms, several processes operate that can cause even more warming.
For instance, snow and ice melt away in a warmer world, exposing dark ground that absorbs the Sun’s heat rather than reflecting it. Similarly, more water vapour evaporates from the surface, and since water vapour is a greenhouse gas this traps even more heat.
The oceans actually slow climate change, because carbon dioxide dissolves into them from the air. But warmer oceans can hold less carbon dioxide, leaving ever more in the atmosphere.
These feedbacks are relatively well understood, but others are harder to unravel: for example, how changing cloud cover will affect the climate, or when methane locked up in permafrost at the poles will be released. Climate models can only make predictions based on what we know, so as temperatures rise further beyond anything humans have experienced, their predictions become less reliable.
So rather than trying to predict what the climate will do from first principles, we can take another approach: we can look at what has happened in the past.
About 55 million years ago, the Earth experienced one of the fastest temperature rises in its history.
During the “Palaeocene-Eocene Thermal Maximum” (PETM), average sea surface temperatures reached up to 10 °C at the poles, compared to -2 °C today. It was a time when there were palm trees as far north as the Arctic Circle, with absolutely no ice at the poles. Some species flourished in the sweltering heat, while others were wiped out.
Those gases warmed the planet by at least 5 °C and maybe as much as 8 °C
It is clear that greenhouse gases were the main driver. In particular, a massive amount of methane escaped from the seabed into the atmosphere, boosting the greenhouse effect.
It is not clear how the methane escaped. Volcanic eruptions or a comet impact have both been proposed, but the most likely explanation is that the Earth was already gradually warming for some other reason. When it reached a certain temperature, the methane stores under the seabed became unstable.
The PETM shows clear parallels to today’s world. In particular, the pulse of greenhouse gases that set it in motion seems to have been roughly equivalent to what humans could release if we burnt all recoverable fossil fuels. Those gases warmed the planet by at least 5 °C and maybe as much as 8 °C, probably over a few thousand years.
How climate change will affect us
Sea level: predicted to rise by up to 1m by 2100
Extreme weather: intense hurricanes may become more frequent
Drought: more likely in mid-latitude regions, due to a combination of higher temperatures and 30% less rainfall
Flooding: rainfall is predicted to increase in the tropics and at high latitudes, causing more frequent floods
Food production: global crop yields predicted to decrease by over 30% by 2050
Extinctions: species extinctions will become more likely, with 40% of ecosystems affected this century
Human health: increased instances of respiratory problems and infectious diseases
Source: Climate Change 2014 Synthesis Report, IPCC
Is that the limit, or could the planet get even hotter than it was during the PETM?
There is a theoretical mechanism that could massively overheat the Earth: a “runaway greenhouse effect”.
We have already seen that heating the planet releases more greenhouse gases, causing yet more warming. In theory this self-feeding mechanism could become unstoppable, warming the planet by hundreds of degrees.
This has never happened on Earth: we would not be here if it had. But scientists believe that it happened to the nearest planet, Venus, 3-4 billion years ago.
Venus is closer to the Sun than Earth, so it was hotter to begin with. On the surface, temperatures increased so much that all of its liquid water evaporated into the air. This water vapour trapped even more heat, and with no water on the surface there was nowhere to store carbon dioxide.
This led to extreme greenhouse conditions. Eventually all the water vapour was lost to space, leaving Venus with an atmosphere that is 96% carbon dioxide.
The planet now has a global temperature of 462 °C. That is hot enough to melt lead, and makes Venus the hottest planet in the solar system: even beating out Mercury, which is closer to the Sun.
It is almost certain that Earth will succumb to such a catastrophe in a few billion years’ time.
As the Sun gets older, it will slowly run out of fuel and swell up into a red giant star. Eventually it will become so bright that the Earth will no longer be able to dissipate the extra heat out into space. The planet’s surface temperature will increase, boiling the oceans and triggering a runaway greenhouse effect that will end all life as we know it, leaving Earth baking under a thick shroud of carbon dioxide.
It seems that the likelihood of us being steam-cooked is pretty low
However, these changes to the Sun will happen over billions of years, so they are not a pressing problem. The question is, could we trigger a runaway greenhouse effect ourselves?
A study published in 2013 suggests that it is possible, but we would have to release a truly staggering amount of carbon dioxide. This gas currently makes up almost 400 parts per million of the air, up from 280ppm before the Industrial Revolution. To trigger a runaway greenhouse effect, we would have to get the level up to 30,000ppm.
That would take about 10 times more carbon dioxide than could be released if we burned all known fossil fuels. There are other sources of greenhouse gases, like the seabed methane that escaped during the PETM, so we cannot absolutely rule it out. But it seems that the likelihood of us being steam-cooked is pretty low.
Clearly, that does not mean heating up the planet is a safe thing to do. A temperature rise of just a few °C will have all sorts of unwelcome impacts (see "Climate impacts"). In particular, parts of the planet could still get too hot for humans to survive.
In the hottest places on Earth today, like California’s Death Valley, temperatures can reach well over 50 °C. Such heat is dangerous, but with proper care it is survivable. That is because the air is dry, so we can cool ourselves by sweating.
A 12 °C rise in temperature would render half of the Earth’s land area uninhabitable
It can be trickier if the air is both hot and humid, as in tropical jungles. The moisture in the air means our sweat evaporates more slowly, so it is harder to cool down.
The best way to assess the combination of heat and humidity is to measure the “wet-bulb temperature”. This is the temperature a thermometer reads if the bulb is wrapped in a damp cloth and a fan is blowing air over it. If you are sweating, this is the lowest temperature that you could cool your skin to.
Humans have to maintain a core body temperature of 37 °C. To ensure that we can always cool off, we keep our skin at around 35 °C. This implies that a wet-bulb temperature of 35 °C or above, if it was sustained for more than a few hours, would be fatal. Even if we could survive it, we would have to sit still.
Even in the most sweltering tropical rainforests, the maximum wet-bulb temperatures recorded have never exceeded 31 °C. This is because hot and humid air is unstable. It rises and cooler air sweeps in beneath, which is what causes tropical thunderstorms.
But that could change.
Air can only rise if the air around it is cooler and denser. So if climate change heats up the tropics, the air will have to be even hotter and more humid before it starts to rise. A study published in 2010 estimated that, with each 1 °C rise in average global temperature, the maximum wet-bulb temperature will rise by 0.75 °C.
That leads to some intimidating conclusions. A 7 °C rise in global temperatures, which we might well hit by 2200, will render some parts of the globe inhospitable to human life. A 12 °C rise in temperature would render half of the Earth’s land area uninhabitable.
Of course, we might try to adapt by installing huge amounts of air-conditioning equipment. But apart from being colossally expensive, this would imprison people inside buildings for days or weeks at a time.
Even if this never comes to pass, on current trends it is likely that Earth will be 4 °C warmer by the end of this century than it was before the Industrial Revolution, and 3 °C warmer than it is now. That would not directly kill us or render parts of the planet uninhabitable, but it will still create enormous upheaval.
20,000 years ago, the Earth was about 4 °C cooler than it is now. This period is known as the “Last Glacial Maximum”. Ice sheets covered most of Canada and northern Europe, including all of the British Isles.
Since then Earth has warmed up by 4 °C. That was enough to remove the ice sheets from Europe and North America. The meltwater from the retreating ice raised sea levels by tens of metres, drowning shallow lands beneath the waves.
When you consider that, it is easy to imagine what another 4 °C rise in temperature could do to the world around us.