Without a longer-term view, the Paris Agreement will lock in warming for centuries

The Paris climate agreement set a “safe” global warming limit of below 2℃, aiming below 1.5℃ by 2100. The world has already warmed about a degree since the Industrial Revolution, and on our current emissions trajectory we will likely breach these limits within decades.

However, we could still come back from the brink with a massive effort.

But let’s take a closer look at that warming limit. If we accept that 1.5-2℃ of warming marks the danger threshold, then this is true whether it applies tomorrow, in 2100, or some time thereafter. What we need is to stay below these limits for all time.

Put it this way: we wouldn’t be satisfied if the brakes on a new car only worked on the day of purchase, or for two weeks after that – we expect them to keep us safe throughout the car’s lifetime.

The trouble is, limiting warming to well below 2℃ forever is a much harder job.

Millennia matter

Whatever warming we manage to prevent this century, the world will continue to respond to climate change after 2100.

Looking beyond 2100 is often considered irrelevant, given that electoral timescales only operate over several years, and individual development projects over several decades.

However, it is highly relevant to major infrastructure developments, such as overall city planning. Throughout Europe and Asia, the foundations of most city infrastructure date back centuries, or even millennia. Not incidentally, so do most of the supporting agricultural and fisheries traditions and transport routes.

Even the more recent developments in the Americas, Africa and Australia have fundamental roots that date back hundreds of years. Clearly, we need to think beyond the current century when we think about climate change and its impact on civilisation.

The short and the long of it

The climate system is made up of many different components. Some of these respond rapidly to changes, others over much longer timescales.

The components that respond rapidly to the impacts of greenhouse gas emissions include changes in cloud, snow and sea-ice cover, in dust content of the atmosphere, land-surface changes, and so on. Some work almost instantaneously, others over decades. Together these are known as the “transient” response.

Slow-responding components in the climate system include ocean warming, continental ice-sheets and exchanges of carbon between lifeforms, oceans, the sea floor, soils and the atmosphere. These work over many centuries and are known as the “equilibrium” response.

Large amounts of energy are needed to warm up such a large volume of water as the global ocean. The ocean has taken up more than 90% of all the extra heat caused by greenhouse gases emitted since the Industrial Revolution, especially into the upper few hundred metres.

However, the ocean is so vast that it will continue to warm from the top down over many centuries to millennia, until its energy uptake has adjusted to Earth’s new energy balance. This will continue even if no further emissions are made.

Ice sheets on Antarctica and Greenland respond to climate change like an accelerating heavy freight train: slow to start, and virtually unstoppable once they get going. Climate change has been building up since the onset of the Industrial Revolution, but only in recent decades have we started to see marked mass-loss increases from the ice sheets.

The ice-sheet freight train has at last come up to speed and now it will keep on rolling and rolling, regardless of what immediate actions we take regarding our emissions.

Looking to the past

Carbon dioxide levels have reached 400 parts per million (ppm). To find out what this means for the coming centuries, we have to look between 3 million and 3.5 million years into the past.

Temperature reconstructions suggest the world was 2-3℃ warmer than before the Industrial Revolution, which is similar to the expected equilibrium response for the future.

Geological data from the last 65 million years indicate that the climate warms 3-5℃ for every doubling of CO₂ levels.

Before the Industrial Revolution, CO₂ levels were around 280 ppm. Under all but the most optimistic emission scenarios of the Intergovernmental Panel on Climate Change (IPCC), the first doubling (to 560 ppm) is approached or crossed between the years 2040 and 2070.

While we don’t know exactly how high sea level was 3.5 million years ago, we are confident that it stood at least 10 metres higher than today. Most studies suggest sea-level rise around 1m higher than today by 2100, followed by a relentlessly continued rise by some 2m per century. Even a rise of a metre or more by 2100 is murderously high for global infrastructure, especially in developing countries.

Today, some 600 million people live at elevations within 10m of sea level. The same area generates 10% of the world’s total GDP. It is estimated that a sea-level rise of 2m will displace almost 2.5% of the global population.

Even the more immediate impacts of sea-level rise are enormous. In 136 of the world’s largest port cities, the population exposed to flooding is estimated to increase by more than three times by 2070, due to combined actions of sea-level rise, land subsidence, population growth and urbanisation. The same study estimates a tenfold increase in asset exposure.

Back to the future

The eventual equilibrium (long-term) level of warming is up to twice the transient (short-term) level of warming. In other words, the Paris Agreement’s response of 1.5-2℃ by 2100 will grow over subsequent centuries toward an equilibrium warming of 2.3-4℃, even without any further emissions.

Given that we have already reached 1℃ of warming, if the aim is to avoid dangerous warming beyond 2℃ over the long term, we have to avoid any further warming from now on.

We can’t do this by simply stopping all emissions. This is because there is still some warming to catch up from the slower transient processes. To stop any further warming, we will have to reduce atmospheric CO₂ levels to about 350 ppm. Doing so requires both stopping the almost 3ppm rise per year from new emissions, and implementing carbon capture to pull CO₂ out of the atmosphere.

Global warming would be limited to 1-1.5℃ by 2100, and 2℃ over the long term, and in addition ocean acidification would be kept under control. These are essential for containing the impacts of climate change on global ecosystems.

This is the real urgency of climate change. Fully understanding the challenge can help us get to work.