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Wednesday, February 19, 2020

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

Sou | 3:25 PM Go to the first of 19 comments. Add a comment
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. https://doi.org/10.1038/s41558-019-0658-7

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





19 comments:

  1. Well, if its any consolation, here in the UK the general public are now convinced its high time somebody else did something about it.

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    1. Like the EU? /s

      The UK is doing a lot better than Australia. Sorry to hear about the awful floods.

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    2. The floods are the result of a double whammy. First you have climate change messing up our weather. We have more rain in winter, less rain in summer: as the IPCC predicted.

      And secondly, the government is not capable of coordinating a response. A Cobra meeting full of Brexiteers blaming the EU for everything is not going to help.

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    3. I imagine everyone in the UK has been affected in one way or another. Were you directly affected?

      Hope you're okay.

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  2. the problem you always face is "attribution" - for the average person the amount of "data" required to make a causal link is huge

    that an increase risk of a severe flooding events is a statistical artefact of a warmer atmosphere (backed by well understood physics) is hard to convey in a 60 sec media interview

    i just wonder why climate scientist don't use the analogy of tobacco / cancer

    after all no one can link a single cigarette to lung cancer, yet we know cigarettes causes lung cancer - how? (that's a rhetorical q btw)

    to add, I live in Cambridge - was severely inconvenience (i survived!!) by an XR sit in at the Fens Causeway junction a few nights ago - that's another conversation

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  3. Hi, I see you've lowered yourself to comment at Judy's denialsphere. Dpn't feel too dirty... Anyway one thing that I noted was that clown who wrote the article claimed that Glaisher shield max temeperatures were cooler than those recorded in Stevenson screens. That's a total lie. Even the link to Warwick Hughes's paper show taht Glaisher-Stevenson is positive. i.e. Glaisher readings are hotter. Judy just promotes lies now. No excuse for letting that get through.

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    1. You noticed. I'm pleased someone did.

      I washed my hands very thoroughly afterwards and managed to contain the nausea. I'm still apologising to my computer for exposing it to that disgusting drivel.

      Judith is contemptible posting that and other articles full of lies and misinformation. She's complained she couldn't get a job after she quit her tenured post yet she's done everything she can to make sure she's unhireable.

      There's absolutely no excuse for deliberately misleading her readers, not even her own abysmal ignorance about climate science (if that's her excuse).

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    2. I see that Anthonu Banton picked this up at WTFIUWT

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  4. Sounds reasonable. The land warms more quickly than the water and so once equilibrium is reached the land cools and the ocean warms slightly compared to transient states. Question - do they really mean the "land" as in the ground, the trees and other vegetation, etc or do they mean the near surface air? If they mean the land, how are they allowing for land use changes over time affecting surface heating? Or is that too small to bother about?

    Given there is research that suggests Australia's climate is warmer and drier due to vegetation clearing (a signal almost comparable to the average atmospheric temperature rise of the past century) it seems to my eye that land use changes would complicate deriving a surface temperature quite a bit.

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    1. GordonACT - deforestation does affect climate; however, there's no research I'm aware of that suggests all the recent warming in Australia is from land clearing. It wouldn't make sense. If you know of any, do share. You may be confusing local and regional impacts of land clearing with global/continent-wide impacts, and/or mixing up "all" and "part".

      As for land surface temperature, weather stations usually measure temperature from about 1.2 m above the ground to ensure good air circulation.

      http://www.bom.gov.au/climate/cdo/about/airtemp-measure.shtml

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    2. Sorry, I was being a bit more general than that. I was really wondering what they meant by discussing the warmth of "the land". As I understand it, the idea of a warming atmosphere depends on the sun warming the earth which warms the air. The air warms due to the presence of GHGs, so all things being equal more GHGs=warmer air. But the earth surface (the land itself) will also warm to some extent. However I'd have imagined that the air would warm more than the land because we are talking about the radiative effect of well mixed gasses versus highly varied land surfaces. Given that land clearing (which yes, is regional) seems to have a significant effect on local air temps, I am wondering if the authors really mean "the land" or rather mean the air as measured over land.

      In terms of regional climate impacts from land cover changes, these do seem to be fairly widespread and should influence overall aggregated temp data wouldn't they? I mean, when we talk about Australia's average warming we are including thermometers that measure those regional effects as well?

      For example, McAlpine and Syktus estimated that returning vegetation cover to pre-settlement conditions could reduce regional warming by as much as 0.5 to 0.9C (for a continental average of 0.18C). This implies that regional and continental warming has been influenced positively by agricultural land use since settlement.

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    3. Oops, sorry, meant to include this: https://www.nature.com/articles/srep29194

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    4. I agree there's evidence indicating reforestation would help offset warming to some extent - less albedo but more evaporative cooling etc. In the paper you've referred to, the authors estimate that area average warming over the whole continent, for the period 2056-2075 compared to 1986-2005, would be reduced from 3.6C to 3.42C with partial restoration of savannah woodlands in Australia. It's not clear if that's just mean summer temperature or annual, probably summer mean, and that's using RCP8.5. (At lesser GHG concentrations, that would be less, with possibly proportionately less effect.) Local and regional effects would be stronger (for mean summer), especially in south eastern Australia and south western WA. Restoring savannah woodlands would likely also affect precipitation according to the authors.

      Re temperature - surface temperature on land, as I said, refers to the temperature taken by instruments usually about 1.2m above the surface. Temperatures of soil and vegetation are not part of the global mean surface temperature analysis. When looking at trends, anomalies are used. That is, people are interested in the changes in temperature over time more than the actual temperatures, when looking at climate change.

      I don't know that changes in the temperature of soil and vegetation would be vastly different to changes in the air around a metre above the surface though I expect there'd be more hour to hour variation, depending on time of day, season, sunlight/shade, soil type, vegetation type, humidity, wind and various other factors. In my experience, soil/gravel/sand is colder than air on a frosty night and hotter than air on a sunny day.

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    5. GordonACT, when you ask what they meant by "the land" you may be referring to this bit I quoted:

      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.

      There the authors are referring to very warm seasons in many regions of the world (mostly on continents) as measured by temperature records (as described above).

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  5. I guess it's a complex subject, I just find the difference between measuring an air temp and comparing it to a surface temp confusing.

    When the authors say that "...for a given global temperature, most land is significantly warmer in a rapidly warming (transient) case than in a quasi-equilibrium climate" they seem to be referring to air temperatures and they go on to talk about regional impacts, as "...this results in more than 90% of the world’s population experiencing a warmer local climate under transient global warming than equilibrium global warming". So they seem to be talking to regional impacts of air temps.

    However,as you refer in your linked post from 2013, when researchers talk about sea surface temperatures it seems they actually ARE referring to a surface temperature. And the paper you reference from Dong et al suggest that at a global average, a rise in sea surface skin temperature actually contributes most to the warming of air over the land. This makes sense as the ocean surface is unlikely to change rapidly (due to constant overturning) whereas the actual land surface does.

    This suggests to me that taken as a global average over longer periods, sea surface temperatures are the most important factor governing atmospheric temperatures (the warm sea warms the air at a relatively constant rate over more of the earth's surface, thus higher GHG concentrations should raise air temps). If Dong et al are right, this means that land temperatures really are air temperatures of air over land that has been mostly warmed by the ocean, which seems at odds with the claim from King et al.

    On the other hand, actual land surface temperature changes are relevant in considering regional warming over shorter time scales (maybe a decade or less). That is, to my naive eye, any underlying long term warming signal will be most affected by SSTs whereas local/regional warming as measured over time is probably most impacted by hot extremes arising from actual changes in surface temps which is why we see the sorts of effects referred to by McAlpine et al. There are other papers noting the regional warming and drying due to land use changes, while others have noted the contribution of managing land radiative properties in limiting local temperature extremes (eg "Land radiative management as contributor to regional-scale climate adaptation and mitigation" https://www.nature.com/articles/s41561-017-0057-5 and "Warming of hot extremes alleviated by expanding irrigation" https://www.nature.com/articles/s41467-019-14075-4).

    Long story short, if land clearing and agricultural use have a noticeable effect on local increases in IR as well as a reduction in cloudiness and rainfall leading to a warming and drying of local climate as well as increased ranges of hot extremes, should we be sceptical of claims that the total warming signal over land, especially in Australia, is largely CO2 driven? And if we compare apples with apples, that decadal and longer air temps are mostly SST driven, is it correct to conclude that warming over land will tend to overshoot equilibrium?

    I'm not after an answer, I know this wickedly complex stuff. Just pointing out that I find the matter difficult to interpret, especially when people talk about "surface" meaning one thing and then another....

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  6. GordonACT - if you're interested in weather and climate have you considered an on-line introductory course or book? There's a lot to unpack in your comments. Meteorology is a specialist discipline that would take some years to cover in its entirety. Meanwhile, here's a link to a discussion of the energy balance.

    I'll make some comments. I won't try to cover it all. I'm not a meteorologist but this is my take on things. It's far from complete. As you say it's a complex subject - though the main bits aren't too complicated. (Someone will no doubt pipe up if I get anything wrong.)

    The seas and the land are both warming up because of the increase in greenhouse gases and the extra energy being sent to the land as well as the sea. Heat emitted from the land surface warms the air, too. It's not just the oceans that warm or cool the air. Heat in the air is what's stopping the ocean from losing more heat, just like it's stopping the land from losing as much heat as it used to. Think about the heat waves this summer fueled by hot air moving east from the interior of Australia. That air was hotter than the air over the ocean. It was warmed when it passed over the land. (Evaporation helps cool the sea surface.) Have you ever felt a sea breeze?

    Small islands are much more affected by surrounding sea surface temperatures than large land masses, like Australia, the Americas and other continents. Sure, it's all connected, but don't discount the impact of increasing radiation on land. It's important and makes a difference, as Andrew King and his co-authors point out (as discussed in the main article here).

    You're right that sea surface temperatures aren't taken by thermometers a meter above the sea surface, it's impractical and it's the sea surface scientists want to track, not air then waves then wind then sunshine etc. For practical reasons as well as scientific reasons people don't stick thermometers in grass or dirt or sand or snow to measure land surface temperature.

    When talking about changes in temperature, McAlpine et al are also talking about land surface temperature as measured by thermometers about a metre off the ground. They aren't talking about sticking thermometers in the dirt. Also, their estimate of returning some shrubs and trees was a change overall of only 5% - instead of an increase in Australian temperatures of 3.6C by 2056-2075, if there were partial rehab of savannah woodlands the increase would be 3.42C. That's all down to the increase in greenhouse gases.

    So no, there's no need to be sceptical about the greenhouse effect causing temperatures to rise on land as well as sea. Land use changes also contributes to climate change, but when you think about it, land use has been changing for thousands of years yet the earth was slowly cooling. It's only relatively recently (since we started burning fossil fuels) that the planet started warming up with a vengeance.

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    1. I hope you don't mind me droning on one last time. Possibly my previous comment was a bit confusing. I'm not disputing the basic science, I understand that the surface must warm with increasing GHGs. However I think what we are most interested in is the warming of the air. The air warms because the surface warms it and warmer air leads to different weather patterns.

      That makes sense but I still wonder at the wisdom of comparing SSTs with land air temps. They seem to me to be two quite different things. If the air is warmed by the surface, then the true effect of increasing concentrations of GHGs should be broadly reflected in air over the oceans. The oceans should be the largest contributor to a warming atmosphere, simply because there is more sea surface and because it is more stable. If we knew how air temps are changing over the ocean, we'd have a good sense of the "stable over time" effects of increasing CO2 on atmospheric warming.

      Over land we have rather more variables, so I would imagine that over land the air temps are subject to both wider ranges and greater excursions to extremes. We know that the ground is often hotter or colder than the air, often by quite a bit as you note. And you observed, as I did, the extreme swings in temperatures in the recent heatwave conditions of December/January.

      Thinking about that and taking into account the research I have referenced, it seems to me that temperatures of the air over land are strongly influenced by local physical qualities. The various papers I have referenced, and others besides, point to this fact that we can make substantial changes (to a significant proportion of the actual local average temperature) by changing land cover/use. It follows that changes since European settlement must also be doing the same thing. The former can cool the local temps, but the latter must have warmed the local temps. That is, the changes we have wrought in the past 200 years in Australia are MORE likely to warm over land air temps, rather than to cool them. As well, such regional and local changes in extremes are more likely in drought when regional IR increases due to reducing cover from over-grazing activity and loss of soil moisture occur.

      So no, I am not discounting the increasing radiation on land. Really, I am wondering whether land surface IR is the more significant factor. This may not be mostly down to "back radiation" but more to land use changes. If true, then in Australia at least we should be paying as much or more attention to land use management than to domestic energy emissions (which contribute next to nothing to a warming atmosphere).

      Or to paraphrase you, land use has been changing for thousands of years yet it's only relatively recently (about the past century) that we have been causing extraordinary large-scale changes in land use and vegetation cover.

      http://theconversation.com/stopping-land-clearing-and-replanting-trees-could-help-keep-australia-cool-in-a-warmer-future-63654

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    2. Adding to atmospheric greenhouse gases is by far the biggest cause of warming. The main thing wrong in what you've written, GordonACT, is attributing more warming to land clearing than to burning fossil fuel. That's not supported by the science you've referred to.

      Land clearing is not good for all sorts of reasons - destruction of ecosystems, releasing carbon, changing the water cycle. It's not the biggest contributor to global warming. (Planting trees might even warm things up a bit in some places, depending on the species, the region, and what they're replacing, because trees don't reflect back as much light.)

      Deforestation is bad because it releases stored carbon to the air, adding to global warming. Planting trees is good because trees store carbon so reduce CO2. However, even if we plant a zillion trees, unless we stop burning fossil fuel Australia and the world will continue to heat up rapidly.

      If you want to cool your home, plant some trees. If you want to take the edge off the heat in towns, have trees in streetscapes and parks.

      If you want to store carbon, plant forests. If you want to cool the planet, stop burning fossil fuels and chopping down forests.

      I'm all for stopping logging, stopping land-clearing and planting millions more trees, as long as we don't neglect the most pressing need at present, which is to stop burning fossil fuel. We can do both.

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