Models, observations not so far apart on planet's response to greenhouse gas emissions

climate
Credit: public domain

How hot our planet will become for a given amount of greenhouse gases is a key number in climate change. As the calculation of how much warming is locked in by a given amount of emissions, it is crucial for global policies to curb global warming.

It is also one of the most hotly debated numbers in science. Observations in the past decade seem to suggest a value that is lower than predicted by models. But a University of Washington study shows that two leading methods for calculating how hot the planet will get are not as far apart as they have appeared.

In , the is how much the surface air temperature will increase if you double from pre-Industrial levels and then wait a very long time for the Earth's temperature to fully adjust. Recent observations predicted that the climate sensitivity might be less than that suggested by models.

The new study, published April 17 in Nature Climate Change, focuses on the lag time in Earth's response. According to most models of climate change, during the early stages of global the sensitivity to greenhouse gas emissions is relatively small. As the ocean catches up and feedbacks kick in, however, the sensitivity increases and the warming rate speeds up. The new study shows that when this difference is factored in, the observations and climate models are in agreement, with recent observations supporting a previously accepted long-term climate sensitivity of about 2.9 degrees Celsius.

"The key is that you have to compare the models to the observations in a consistent way," said author Kyle Armour, a UW assistant professor of oceanography and atmospheric sciences. "This apples-to-apples approach—where you factor in how long the planet has been adjusting to a change in its atmosphere—shows that climate sensitivity in the models is actually in line with what has been seen in the recent observations."

The planet's temperature takes thousands of years to fully adjust to a shift in the makeup of its atmosphere—the warming Earth has experienced to date is just a taste of what is in store. Early climate studies suggested that if the amount of carbon dioxide in the atmosphere doubled from pre-Industrial levels (we're now about 1.4 times) the planet would eventually warm by about 3 degrees C, with possible values as high as 5 or 6 degrees C.

But recent observations of warming so far and emissions to date have suggested that climate sensitivity may be just under 2 degrees Celsius, with a maximum possible value of 4 degrees C.

"If true, this really would be a shift in our understanding of the long-term climate sensitivity," Armour said. For the new study, Armour looked at 21 leading run with increasing carbon dioxide. He focused on the warming rate compared to carbon dioxide levels, or climate sensitivity, in the early stages compared with in the late stages. The late-stage sensitivity across all the models was an average of 26 percent higher than the early-stage values. When factoring in that today's observations are for the early stages of warming, the recent observations support a climate sensitivity of 2.9 degrees Celsius.

"There have been a lot of other papers that looked at the reasons for the changes in climate sensitivity over time," Armour said. "This paper was the first attempt to quantify the effect across all the comprehensive models we use for climate prediction."

The situation can be likened to pressing the gas pedal on a car, but the mass of the vehicle takes a while to get rolling. If the driver floors the gas pedal, it can be tricky to calculate the car's final speed based on its initial reaction.

In the Earth system, the ocean temperatures around Antarctica and in the eastern Pacific Ocean have not risen in recent decades. Armour's previous research showed that deep, slow currents mean seawater touched by will take centuries to reach the surface of the Southern Ocean. Similar but less extreme, currents reaching the eastern tropical Pacific from below the surface have also not seen daylight for decades.

Eventually, water touched by a warmer atmosphere will reach the eastern tropical Pacific and later the Southern Ocean. Warming in these regions will then activate feedbacks that will kick the planet's warming into a higher gear."Currently we don't have any evidence that the models are too sensitive compared to the observations," Armour said. "The models appear to be in line with the observed range of warming."

The various show a wide range of values between the early-stage and late-stage sensitivities. Armour and students are exploring why these differences between the models exist, in order to improve them and better how climate shifts over time.


Explore further

El Nino reveals impact of global warming on marine production

More information: Energy budget constraints on climate sensitivity in light of inconstant climate feedbacks, Nature Climate Change (2017). nature.com/articles/doi:10.1038/nclimate3278
Journal information: Nature Climate Change

Citation: Models, observations not so far apart on planet's response to greenhouse gas emissions (2017, April 17) retrieved 19 July 2019 from https://phys.org/news/2017-04-planet-response-greenhouse-gas-emissions.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
57 shares

Feedback to editors

User comments

Apr 17, 2017
I see that Nature is already ignoring its announcement that it will only publish replicable studies.

Apr 17, 2017
I see that Nature is already ignoring its announcement that it will only publish replicable studies.


Fortunately, the rest of us here don't suffer from trollvision, as you, HalfAnnie.

You should see a professional about getting that deficit addressed.

Apr 18, 2017
Nothing in the past suggests the climate is intrinsically stable. It's just Homo sapiens has been living in a very special time for 10,000 years. We are about to find out just how special.

Apr 18, 2017
Nothing in the past suggests the climate is intrinsically stable.

However, nothing in the past suggests that climate has ever altered this quickly before. (Yesm climate has altered on this scale before - but over a much, much, MUCH larger time scale)

Apr 18, 2017
Indeed. This paper shows that the sensitivity is, far from the 1.2C the luke-warmists tout, likely to be 3C and more.

Because the climate is not stable. The feedbacks can be strongly positive and more powerful than the forcing.

The sensitivity suggested by the models seem to be echoed by the glaciologists looking at the Arctic and Antarctic.

Apr 18, 2017
Models are good. And since the age of the computer, very good. Though not perfect, they are quite representative. Engineers count on them to figure out aero of race cars; Virgin F1 designed the aero of their first car entirely on the computer. Ship builders model hull design. Skyscraper architects model movement at height due to wind, and so on. Time has shown that they get it right. At a social level we accept and count on them 100% for construction because the math works.

What is not easy to model is how living things will respond. So when a thousand scientists in a hundred papers say changing the atmospheric gas mixture by X will result in Y temperature at a given time lag, you can take it to the bank. Don't think for as second that lay opinions to the contrary carry equal weight and likelihood. What is less clear, is how individuals and society will respond to the physical reality. This makes it hard to responsibly calculate the risk.

Apr 18, 2017
When you constantly tweak your models to account for variances between the predicted and the observed, it is no wonder the observations are not so far apart.
The 2005 UNEP predictions claimed that, by 2010, some 50 million "climate refugees" would be frantically fleeing from the Caribbean and low-lying Pacific Islands. However, not only did the areas in question fail to produce a single "climate refugee," by 2010, population levels for those regions were actually still soaring by 2010.
(Time to tweak the prediction machine!)
There are many more examples of poor model predictions. I know someone is going to tell me that their models are much better today, but that is exactly my point. "Tweaking' the model until is agrees with the observation is not good science.

Apr 18, 2017
This is real.

How many Deniers fell for the draft-dodgers screaming "WMD!", and think we are just as emotionally-vulnerable? Sorry, but we think before we send our sons and daughters to become killers of those who had done nothing to us.

How did that work out? Why are they still following those fools?

Apr 18, 2017
The 2005 UNEP predictions claimed that, by 2010, some 50 million "climate refugees" would be frantically fleeing from the Caribbean and low-lying Pacific Islands. However, not only did the areas in question fail to produce a single "climate refugee," by 2010, population levels for those regions were actually still soaring by 2010.


The 50 million figure was for the whole globe and was based on this paper: -

http://www.osce.o...oad=true

No mention of Caribbean and low-lying Pacific Islands on their own. To highlight how wrong your claims are the population of the Caribbean in 2009 was 39.17 million and the World Bank's Pacific Island member countries have a population of about 2.3 million people, so nobody would claim 50 million refugees from those areas alone.

Do you have any evidence to back up your claims?


Apr 18, 2017
'Tweak' …slang word derived from "Pinch" which describes an unpleasant narrowing. When a person applies a "Pinch" there is often a sadistic or malicious emotional association. It's an unhelpful word; a misleading emotional euphemism.

This article describes further substantiation of earlier work. It follows on previous news that the earliest mathematics of Arrhenius et al., and the first crude models developed during the advent of planetary exploration, have proven essentially true. Borne out by subsequent observations on Earth.

Models are just Calculus. Math for solving outcomes involving multiple variables. When real life proves them right, the mathematician yawns.

New, overlooked variables included? Better Model.


Apr 18, 2017
SamB, "Tweaking' the model until is agrees with the observation is not good science." is completely incorrect. Faking the observation to match your model and then claiming your model is correct is bad science. Models/theories are constantly tweaked to better match empirical evidence and thus boost their power of prediction.

Apr 20, 2017
I will add the quotes: "We don't trust climate models yet to predict specific episodes of extreme weather because the models are too coarse...."However, the models do faithfully reproduce large scale patterns of temperature change." (Dim Coumou, Kai Kornhuber of Potsdam Institute for Climate Impact Research, Germany).

Apr 23, 2017
I see that Nature is already ignoring its announcement that it will only publish replicable studies.

I see that you continue to ignore the way science actually works, and then substitute your ignorance and paranoid delusions for reason.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more