Hot enough for you? The faster it warms the hotter it gets

An interesting if ominous paper was recently published in Nature Climate Change. It came out just before Christmas, at the height of the holiday season here in Australia while fires were raging. For some weeks I’ve been meaning to write about it. That moment has finally arrived.

The authors of the Nature Climate Change paper, Andrew D. King, Todd P. Lane, Benjamin J. Henley and Josephine R. Brown (from The University of Melbourne) tell us that it’s up to us to a large degree (excuse the word play). We know that already, and we also know that recent history and current weather-related events in Australia, the UK, Africa and elsewhere demonstrate we’ve not yet been willing to take enough action.

However the authors weren’t writing about our reluctance to do enough to save ourselves. They were in effect exploring what will happen if we can slow down global warming compared to if we let it continue to warm as quickly as it is. It probably won’t surprise HotWhopper readers that the rate of warming makes quite a difference.

You may have seen the excellent series of articles in the Washington Post late last year: 2°C: Beyond the Limit: Dangerous new hot zones are spreading around the world. These articles were describing the impact of global warming in different parts of the world, where warming has already exceeded 2°C. On average, the world has warmed maybe a bit more than 1.1 °C above pre-industrial temperatures; however, some places are warming faster than others. This includes some ocean areas as well as land areas. Most of us live on land, so what happens on land is of particular interest.

As you may know, when the world is heating up, the land heats up faster than the ocean. This is because very large bodies of water have to absorb a huge amount of energy (heat) for the temperature to rise much whereas it doesn’t take much energy to warm up the land surface. Specific heat capacity is a measure of a substances capacity to absorb energy compared to how hot it gets. It is defined as the amount of heat needed to raise the temperature of 1 gram of a substance 1 degree Celsius (°C). Water needs a lot of heat to raise its temperature. Land doesn’t need nearly as much.

The faster it warms the hotter we get

The authors of King19 decided to look at the different effects around the world when the climate is warming compared to when it’s more or less stable. It’s clear that the rate of warming makes a big difference. Their results indicate that as it warms it gets hotter on land compared to how hot it would be on land at the same global average temperature when the climate is in equilibrium. In other words, the land gets hotter the faster it warms. (I’m putting words into the authors’ mouths here. It’s fair IMO.)

A difference between 1.5 °C warmer at equilibrium and 2 °C warmer at equilibrium would certainly be noticed. However, it’s the difference between how hot it gets during the transition to equilibrium that we’d notice a whole lot more.

As described in the abstract:

…more than 90% of the world’s population experiencing a warmer local climate under transient global warming than equilibrium global warming. Relative to differences between the 1.5 °C and 2 °C global warming limits, the differences between transient and quasi-equilibrium states are substantial. For many land regions, the probability of very warm seasons is at least two times greater in a transient climate than in a quasi-equilibrium equivalent. In developing regions, there are sizable differences between transient and quasi-equilibrium climates that underline the importance of explicitly framing projections.

Transient vs quasi-equilibrium

The transient state refers to the state while change is happening, while the world is getting hotter. The quasi-equilibrium state is, as you can probably work out, a state where the climate is unchanging, is more or less steady.

Say the world is on average 2 °C hotter than in pre-industrial times. Now if it’s come from 1 °C hotter (like now) and just hit 2 °C hotter on its way to 3 °C hotter, it’s in a transient state. If it’s been sitting at around 2 °C above and not varying much over time (no major forcings, just some internal variability) it’s in a state of equilibrium. [The term “quasi-equilibrium” is used because that state is based on CMIP5 models for 2300 (mid-range greenhouse gas emissions – ECP4.5). The climate of the 2300s is not in full equilibrium, but getting close, hence “quasi-equilibrium”.]

The land gets hotter faster until we reach equilibrium, then it cools a bit

What does this all mean? It means much of where we live will get a lot hotter until we stop warming the planet, then the land will cool down a bit as it reaches equilibrium with the ocean. That is, even while global surface temperature is steady, after warming stops, the hotter areas of land will cool a bit and the cooler parts of the oceans will keep warming a bit until equilibrium is reached.

The lead author, Andrew King, put it simply in an email, saying:

“For a given level of global warming, in a transient climate most land areas are warmer and experience more heatwaves than in an equivalent equilibrium climate with the same global temperature. So, if we were to hold the global temperature constant then most land areas would cool over time.” 

Land vs oceans 

Consider what happens at 1.5 °C and 2 °C while the world is heating up, compared to the situation if the global surface temperature is more or less steady. While the world is heating up, land warms quickly and is out of sync with the oceans. In the transient 1.5 °C world, the continental land regions are warmer than they would be in a quasi-equilibrium 2 °C world. It’s different for the slow warming oceans. With slow-warming ocean areas, the transient 2 °C world is cooler than the quasi-equilibrium 1.5 °C world.

The impact is illustrated below. The maps show the difference in temperature between transient warming and how warm it would be in a stable climate. Some of the oceans are cooler while most of the land is warmer in the transient climate compared to a climate at equilibrium.

Move the arrow to the right to see what happens in June to August and in the December to February period.

May 18
April 18

Figure 1 | Transient minus quasi-equilibrium difference. In a transient climate, where the world is rapidly warming, land areas are warmer and ocean areas cooler than in a stabilised climate. These two figures shows the pattern of temperature difference between a transient scenario relative to an equilibrium climate for both the June-August period (l) and December-February period (r). Source: Article by Dr Andrew King in Pursuit (The University of Melbourne)

More heat waves and hot seasons as the world warms

The research also indicates that there’ll be a lot more severe heat events while the world is heating up than there will be after some sort of steady state temperature is reached. Where a hot season may be a one-in-ten year event in a quasi-equilibrium climate, in a transient climate this could be a one-in-five year event.

From the paper (my para break):

This is particularly true in boreal summer where, for almost all land regions of the world, the likelihood of a hot season is significantly higher in a transient climate compared with the equivalent quasi-equilibrium state. Regions with at least a doubling in the probability of hot summers in a transient climate compared with a quasi-equilibrium world of equivalent global warming encompass major cities including New York City, Istanbul, Baghdad, Seoul and Tokyo.

Although this is a global-scale analysis, one could infer that for these densely populated locations, the heat-related impacts of human-induced climate change would be lessened in a stabilized climate compared with a rapidly warming climate at the same level of global warming.

It’s the coming generations who’ll be hardest hit

Think what that means while global heating continues. If it keeps warming as it has, then the land will warm up faster, there will be more and worse heat waves, more severe fires, worse droughts etc. After the new climate approaches equilibrium, things should settle down a bit. The biggest upheaval will be during the transition. If society can survive that, generations far into the future will have a fighting chance.

This is an important piece of work because policy and planning people need to think about what’s going to happen in their region over coming decades. If they merely focus on the likely scenario should the global temperature stabilise at 1.5 °C, 2 °C, 3 °C and hotter, they’ll not be prepared for what happens along the way. Like the Australian Government this year, countries could be woefully unprepared for the changes to come during the transition to a new climate equilibrium.

The only comment I’ll add is that the researchers discussed 1.5 °C and 2 °C. The world has not yet taken sufficient action to limit warming to 2 °C. We are still heading for 3 °C and hotter over the coming decades. It’s time to take action and slow things down to limit the number and frequency of worse disasters than the ones we’re now experiencing.

I’ll leave you with this quote from the Pursuit article by Andrew King, which to my way of thinking really drives the point home about the importance of stabilising the climate:

In fact for some areas of the world, if we were to achieve the Paris Agreement goal of stabilising the climate at 1.5°C global warming, we would experience cooler average summer temperatures and fewer hot summers than we do in our current rapidly warming world after 1.1°C of human-induced global warming.

References and further reading

King, Andrew D., Todd P. Lane, Benjamin J. Henley, and Josephine R. Brown. “Global and regional impacts differ between transient and equilibrium warmer worlds.” Nature Climate Change 10, no. 1 (2020): 42-47.

2°C: BEYOND THE LIMIT: Dangerous new hot zones are spreading around the world a series of articles at The Washington Post, September to December 2019