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Isla Nublar
28 June 2016 16:08

Scientists find common ground over climate sensitivity

Roz Pidcock

Roz Pidcock

Roz Pidcock

Roz Pidcock

28.06.2016 | 4:08pm
Climate sensitivityScientists find common ground over climate sensitivity

A new paper helps to shed light on one of the biggest questions in climate science: how much the climate will warm in future?

The answer to this depends a lot on something scientists call the “climate sensitivity” – a measure of how much the climate warms in response to greenhouse gases.

Until now, scientists have been grappling with how to reconcile the fact that different ways to estimate the climate sensitivity have, so far, come up with quite different answers.

This uncertainty has never been a reason to question whether climate change will be serious or to delay action to tackle emissions, though it has often be misused by climate skeptics this way.

But a question mark over the value of climate sensitivity has meant that projections of future temperature rise are less precise than scientists would like. It also makes it harder to gauge our chances of staying below a given temperature limit, such as 2C above pre-industrial levels.

A new paper published in Nature Climate Change says there is, in fact, no disagreement between the different methods after all. In reality, they measure different things and once you correct for the fact that the historical temperature record underestimates past warming, the gap closes.

The implications are significant since it suggests we’ve seen around 0.2C more warming than previously thought, says co-author Dr Ed Hawkins in his Climate Lab Book blog.

Infographic showing where the heat from global warming comes from

Credit: Rosamund Pearce, Carbon Brief.

A sensitive question

The new paper looks at two primary ways of estimating transient climate sensitivity, defined as the amount of near-surface warming resulting from a doubling of carbon dioxide.

One way is to compare observations of how much the land, ocean, ice and atmosphere have warmed over the industrial period to how greenhouse gases and other factors that influence temperature, known as forcings, have changed in the same time.

But estimates of climate sensitivity from these so-called “energy budget models” tend to be lower than those simulated by climate models, raising the question of whether models are overestimating the scale of the climate response to greenhouse gases.

Climate models work differently, by calculating the warming expected from greenhouse gases and all processes in the climate system known to amplify or dampen the speed of warming, known as feedbacks.

In a News and Views article accompanying the new research, Prof Kyle Armour from the University of Washington, says:

“The widest range of climate sensitivity supported by recent observations is 1.0-4.0C, with a best estimate at around 2.0C. At face value, this suggests that models, with a range of 2.0–5.6C, are altogether too sensitive.”

“Apples to oranges”

The new study argues that issues with the way global temperatures have been measured historically mean that they underestimate the total warming over the industrial period.

First, there are places in the world where temperature data doesn’t exist. Importantly, the most poorly sampled areas are also the fastest warming. In the HadCRUT4 dataset, it is well known that gaps exist over remote parts of the Arctic, for example.

By reducing the geographical coverage in the models to match the observational record – a process known as “masking” – the scientists worked out that imperfect geographical coverage effectively pushed observation-based estimates of warming down by about 15%.

Secondly, the techniques scientists use for making temperature measurements have advanced over time. Hawkins says:

“Historical weather observations include air temperatures measured over land areas and sea surface temperatures in ocean regions, but only where there were existing thermometers (& ships).”

But “blending” these different – and often sparse – measurements to calculate a global average temperature has unwittingly caused problems, Armour writes:

“[S]hip-based measurements are actually of the ocean’s surface layer, which has been warming at a slightly slower rate than the air just above.”

Since climate models simulate only air temperatures, this is not a like-for-like comparison.

Adapting the models to account for the difference between seawater and near-surface air temperatures in the models allowed the scientists to see what difference the instrumental differences make. Hawkins explains:

“As we cannot travel back in time to take additional historical observations, the only way to perform a like-with-like comparison is by treating the models differently.”

Doing so, suggested that the instrumental changes within the HadCRUT4 dataset underestimated global temperature change over the industrial period by a further 9%, to a total of 24% when combined with the effect of incomplete observations.

The difference this is not trivial, says Hawkins:

“According to the CMIP5 simulations, more than 0.2°C of global air temperature change has been ‘hidden’ due to our incomplete observations and use of historical sea surface temperatures.”
Chart showing average temperature change relative to 1861–1880 for CMIP5 models

Average temperature change relative to 1861–1880 for CMIP5 models with spatially complete air temperature (red), after “blending” and “masking” to account for incomplete observations and use of historical sea surface temperatures (blue). Credit: Dr Ed Hawkins, Climate Lab Book.

Based on their calculations, the scientists’ best estimate for how much the observational record underestimates transient climate sensitivity is 1.7C, with a range of 1.0-3.3C.

Scaling up previous estimates based on the imperfect observational record brings them in line with the best estimate from models of 1.8C, with a range of 1.2-2.4C. Or Armour says:

“Climate sensitivity estimates have now been reconciled.”

Prof Piers Forster from the University of Leeds is cautious about overinterpreting the results too soon. He tells Carbon Brief:

“I think it just emphasises that [climate sensitivity] estimates based on historical observations remain quite large. There may not be a discrepancy between the various types of estimates. Or we may find physical reasons why they differ, as other papers suggest. In my mind, the jury is still out on this one.”

In other words, questions about climate sensitivity are complicated and won’t be solved by any single bit of research.

Further questions

The impact that gaps in the temperature record have on climate sensitivity is likely to diminish in future as observational coverage improves, the study notes.

But the problem will still remain for the historical period unless scientists can rescue additional, currently undigitised, weather observations, says Hawkins.

The study raises another interesting point: which global mean temperature is relevant for informing policy? The imperfect record based on observations, or the model record with gaps filled in? On this point, Hawkins says:

“If it is decided that climate targets refer to the latter, then the warming is actually 24% (9-40%) larger than reported by HadCRUT4. And that is a big difference, especially when considering lower global temperature targets.”
Main image: Isla Nublar. Credit: Mark Freeth/Flickr.
Sharelines from this story
  • Scientists find common ground over climate sensitivity
  • Philip Haddad

    It should be noticed that global warming started around 1920. It should also be noted that CO2 concentration was only 300 ppm at that time. CO2 did not start global warming! The burning of fossil fuels releases heat (as well as CO2) and in 1920 was about 1 terawatt, enough to over-ride the cooling trend of 1000 years that had preceded it. Energy use has been steadily increased surpassing 16 TW in 2008. Since 80% of our energy comes from fossil fuels it is only natural that CO2 has also risen steadily. CO2 is beneficial and attempts to remove it are counter-productive. Conversion of CO2 through photosynthesis removes 4800 btus of solar energy per pound that other wise becomes heat. Since trillions of tons of glaciers have already melted, the albedo (reflected heat) may have decreased enough that the increased solar heat will perpetuate the cycle regardless of any efforts to halt global warming.

    • MikeH

      Total nonsense.

      It is debunked here by Geophysics Professor Ray Pierrehumbert

      >Wherever it comes from, waste heat is not usually taken into account in global climate calculations for the simple reason that it is utterly trivial in comparison to the heat trapped by the carbon dioxide that is released when you burn fossil fuels to supply energy. For example, that 6 trillion Watts of waste heat from coal burning would amount to only 0.012 Watts per square meter of the Earth’s surface. Without even thinking very hard, you can realize that this is a tiny number compared to the heat-trapping effect of CO2. As a general point of reference, the extra heat trapped by CO2 at the point where you’ve burned enough coal to double the atmospheric CO2 concentration is about 4 Watts per square meter of the Earth’s surface — over 300 times the effect of the waste heat.

      • Philip Haddad

        How do you know what the real heat trapping effect of CO2 is? What is the effect of an incremental increase of 100 ppm? Without thinking at all you can ignore the fact that global warming started around 1920 when CO2 was 300 ppm and continued to rise as energy use rose. Without thinking at all you can ignore the fact that the heat produced from energy use is more than four times the amount that can be attributed to the rise in air temperature. Why not? The Kyoto “scientists” ignored it so why not you?

  • Occupy My Brain

    You are overlooking the heat generated by cell towers (registered & unregistered) as well as satellite RFEFs (known and unknown) – compound that with heat + metal generated downward and outward you are heating and drying the earth’s surface as well as any metallic structures above and into the ground. The recommended frequencies for these towers and satellites is not monitored and the US Government has ignored scientific evidence showing the environmental and human impacts due to pressure from the telecommunications industries. Our guidelines here in the USA are so overblown compared to other Nations in the world. Has that been addressed or is it going to be?

    • MikeH

      Even more ridiculous than the previous comment.

  • As the illustration shows, with only 1% of the heat going into the atmosphere and 93% going into the oceans the oceans have a huge effect on the temperature of the planet. We know very little about the oceans and have only started taking records from so there is a huge gap in our knowledge. Most of the biggest threats to our way of life are based on what happens in the oceans so we need to increase the research.

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