What is “committed warming”? Climate scientist explains why global warming can continue long after emissions end

Today, few people question the reality that humans alter the earth’s climate. The real question is: how quickly can we stop, or even reverse, the damage?

Part of the answer to this question lies in the concept of “engaged warmingalso known as “pipeline warming”.

It refers to future increases in global temperatures that will be caused by greenhouse gases that have already been emitted. In other words, if the transition to clean energy happened overnight, how much more warming would ensue?

Earth’s energy balance is out of balance

Humans cause global warming when their activities emit greenhouse gases, which trap heat in the lower atmosphere, preventing it from escaping to space.

Before people started burning fossil fuels to power factories and vehicles and raising methane-emitting livestock in nearly every arable region, Earth’s energy budget was roughly balanced. About the same amount of energy coming from the Sun as was coming out of it.

Today, the increase in carbon dioxide concentrations in the atmosphere is more than 50% higher than they were at the dawn of the industrial age, and they are trapping more of that energy.

Earth’s delicate energy balance. California Academy of Sciences.

These emissions of carbon dioxide, along with other greenhouse gases such as methane, and offset by some aspect of aerosol air pollution, trap energy equivalent to the detonation of five Hiroshima-type atomic bombs per second.

With more energy flowing in than out, the Earth’s heat energy increases, raising the temperature of the land, oceans and air and melting the ice.

Heating in the pipeline

The effects of tampering with Earth’s energy balance take time to manifest. Think about what happens when you fully open the hot water tap on a cold winter’s day: the pipes are full of cold water, so it takes time for the hot water to reach you – from where? the term “heating of pipes”. The warming has not yet made itself felt, but it is in preparation.

There are three main reasons why Earth’s climate is expected to continue to warm after emissions stop.

First the main contributors to global warming – carbon dioxide and methane – persist in the atmosphere for a long time: about 10 years on average for methane, and 400 years for carbon dioxide, with some molecules remaining for millennia. Thus, stopping emissions does not translate into instantaneous reductions in the amount of these heat-trapping gases in the atmosphere.

Second, some of this warming has been offset by human-made emissions of another form of pollution: sulfate aerosols, tiny particles emitted by the burning of fossil fuels, which reflect sunlight back into space. . Over the past century, this overall dimming has been masking the warming effect greenhouse gas emissions. But these and other man-made aerosols also harm Human health and the biosphere. Removal of these and short-lived greenhouse gases results in a few tenths of a degree of additional warming for about ten years, before reaching a new equilibrium.

Finally, the Earth’s climate takes time to adapt to any change in the energy balance. About two-thirds of the Earth’s surface is water, sometimes very deep water, which is slow to absorb excess carbon and heat. So far, more than 91% of the heat provided by human activitiesand about a quarter of the excess carbon, went to the oceans. While land dwellers may be grateful for this buffer, the extra heat contributes to sea level rise across thermal expansion and also marine heat waveswhile the extra carbon makes the ocean more corrosive to many shell organisms, which can disrupt the ocean food chain.

The Earth’s surface temperature, driven by the imbalance of radiant energy at the top of the atmosphere, and modulated by the enormous thermal inertia of its oceans, is still catching up with its biggest control button: carbon dioxide concentration.

How much warming?

So how much committed warming do we expect? There is no clear answer.

The world has already warmed more than 1.1 degrees Celsius (2 F) above pre-industrial levels. Nations around the world agreed in 2015 to try to stop the global average from rising more than 1.5C (2.7F) to limit the damage, but the world was slow to react.

Determining the magnitude of future warming is complicated. Several recent studies use climate models to estimate future warming. A study of 18 Earth system models found that when emissions were cut, some continued to warm for decades or even hundreds of years, while others began to cool rapidly. Another study, published in June 2022, found a 42% chance the world has already committed to 1.5 degrees.

The amount of warming matters because the dangerous consequences of global warming do not simply increase in proportion to global temperature; they usually increase exponentially, especially for food production threatened by heat, drought and storms.

Moreover, the Earth has tipping points that could trigger irreversible changes in fragile parts of the Earth system, such as glaciers or ecosystems. We won’t necessarily know right away when the planet has passed a tipping point, as these shifts are often slow to manifest. This system and other climate-sensitive systems are the basis of the precautionary principle of limiting warming to less than 2°C (3.6°F) and preferably 1.5°C.

The heart of the climate problem, rooted in this idea of ​​committed warming, is that there are long lags between changes in human behavior and climate change. While the precise amount of committed warming is still controversial, evidence shows that the surest path is to urgently transition to a zero-carbon economy, fairer economy that generates far fewer greenhouse gas emissions.

Teresa H. Sadler