Yes, a few climate models give unexpected predictions – but the technology remains a powerful tool


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Nerilie Abram, Australian National University; Andrew King, The University of Melbourne; Andy Pitman, UNSW; Christian Jakob, Monash University; Julie Arblaster, Monash University; Lisa Alexander, UNSW; Sarah Perkins-Kirkpatrick, UNSW; Shayne McGregor, Monash University, and Steven Sherwood, UNSW

The much-awaited new report from the Intergovernmental Panel on Climate Change (IPCC) is due later today. Ahead of the release, debate has erupted about the computer models at the very heart of global climate projections.

Climate models are one of many tools scientists use to understand how the climate changed in the past and what it will do in future.

A recent article in the eminent US magazine Science questioned how the IPCC will deal with some climate models which “run hot”. Some models, it said, have projected global warming rates “that most scientists, including the model makers themselves, believe are implausibly fast”.


Read more: Monday’s IPCC report is a really big deal for climate change. So what is it? And why should we trust it?


Some commentators, including in Australia, interpreted the article as proof climate modelling had failed.

So should we be using climate models? We are climate scientists from Australia’s Centre of Excellence for Climate Extremes, and we believe the answer is a firm yes.

Our research uses and improves climate models so we can help Australia cope with extreme events, now and in future. We know when climate models are running hot or cold. And identifying an error in some climate models doesn’t mean the science has failed – in fact, it means our understanding of the climate system has advanced.

So lets look at what you should know about climate models ahead of the IPCC findings.

What are climate models?

Climate models comprise millions of lines of computer code representing the physics and chemistry of the processes that make up our climate system. The models run on powerful supercomputers and have simulated and predicted global warming with remarkable accuracy.

They unequivocally show that warming of the planet since the Industrial Revolution is due to human-caused emissions of greenhouse gases. This confirms our understanding of the greenhouse effect, known since the 1850s.

Models also show the intensity of many recent extreme weather events around the world would be essentially impossible without this human influence.

 

 

 

Scientists do not use climate models in isolation, or without considering their limitations.

For a few years now, scientists have known some new-generation climate models probably overestimate global warming, and others underestimate it.

This realisation is based on our understanding of Earth’s climate sensitivity – how much the climate will warm when carbon dioxide (CO₂) levels in the atmosphere double.

Before industrial times, CO₂ levels in the atmosphere were 280 parts per million. So a doubling of CO₂ will occur at 560 parts per million. (For context, we’re currently at around 415 parts per million).

The latest scientific evidence, using observed warming, paleoclimate data and our physical understanding of the climate system, suggests global average temperatures will very likely increase by between 2.2℃ and 4.9℃ if CO₂ levels double.

The large majority of climate models run within this climate sensitivity range. But some don’t – instead suggesting a temperature rise as low as 1.8℃ or high as 5.6℃.

It’s thought the biases in some models stem from the representations of clouds and their interactions with aerosol particles. Researchers are beginning to understand these biases, building our understanding of the climate system and how to further improve models in future.

With all this in mind, scientists use climate models cautiously, giving more weight to projections from climate models that are consistent with other scientific evidence.

The following graph shows how most models are within the expected climate sensitivity range – and having some running a bit hot or cold doesn’t change the overall picture of future warming. And when we compare model results with the warming we’ve already observed over Australia, there’s no indication the models are over-cooking things.

Rapid warming in Australia under a very high greenhouse gas emission future (red) compared with climate change stabilisation in a low emission future (blue). Author provided.

What does the future look like?

Future climate projections are produced by giving models different possibilities for greenhouse gas concentrations in our atmosphere.

The latest IPCC models use a set of possibilities called “Shared Socioeconomic Pathways” (SSPs). These pathways match expected population growth, and where and how people will live, with plausible levels of atmospheric greenhouse gases that would result from these socioeconomic choices.

The pathways range from low-emission scenarios that also require considerable atmospheric CO₂ removal – giving the world a reasonable chance of meeting the Paris Agreement targets – to high-emission scenarios where temperature goals are far exceeded.


Nerilie Abram, based on Riahi et al. 2017, CC BY-ND

Ahead of the IPCC report, some say the high-emission scenarios are too pessimistic. But likewise, it could be argued the lack of climate action over the past decade, and absence of technology to remove large volumes of CO₂ from the atmosphere, means low-emission scenarios are too optimistic.

If countries meet their existing emissions reduction commitments under the Paris Agreement, we can expect to land somewhere in the middle of the scenarios. But the future depends on our choices, and we shouldn’t dismiss any pathway as implausible.

There is considerable value in knowing both the future risks to avoid, and what’s possible under ambitious climate action.


Read more: The climate won’t warm as much as we feared – but it will warm more than we hoped


Wind turbines in field
The future climate depends on our choices today. Unsplash

Where to from here?

We can expect the IPCC report to be deeply worrying. And unfortunately, 30 years of IPCC history tells us the findings are more likely to be too conservative than too alarmist.

An enormous global effort – both scientifically and in computing resources – is needed to ensure climate models can provide even better information.

Climate models are already phenomenal tools at large scales. But increasingly, we’ll need them to produce fine-scale projections to help answer questions such as: where to plant forests to mitigate carbon? Where to build flood defences? Where might crops best be grown? Where would renewable energy resources be best located?

Climate models will continue to be an important tool for the IPCC, policymakers and society as we attempt to manage the unavoidable risks ahead.The Conversation

Nerilie Abram, Chief Investigator for the ARC Centre of Excellence for Climate Extremes; Deputy Director for the Australian Centre for Excellence in Antarctic Science, Australian National University; Andrew King, ARC DECRA fellow, The University of Melbourne; Andy Pitman, Director of the ARC Centre of Excellence for Climate Extremes, UNSW; Christian Jakob, Professor in Atmospheric Science, Monash University; Julie Arblaster, Chief Investigator, ARC Centre of Excellence for Climate Extremes; Chief Investigator, ARC Securing Antarctica’s Environmental Future; Professor, Monash University; Lisa Alexander, Chief Investigator ARC Centre of Excellence for Climate Extremes and Professor Climate Change Research Centre, UNSW; Sarah Perkins-Kirkpatrick, ARC Future Fellow, UNSW; Shayne McGregor, Associate professor, Monash University, and Steven Sherwood, Professor of Atmospheric Sciences, Climate Change Research Centre, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.

IPCC says Earth will reach temperature rise of about 1.5℃ in around a decade. But limiting any global warming is what matters most


Dave Hunt/AAP

Michael Grose, CSIRO; Malte Meinshausen, The University of Melbourne; Pep Canadell, CSIRO, and Zebedee Nicholls, The University of Melbourne

Of all the troubling news in the Intergovernmental Panel on Climate Change (IPCC) report out on Monday, one warning will surely generate the most headlines: under all scenarios examined, Earth is likely to reach the crucial 1.5℃ warming limit in the early 2030s.

As the report makes clear, global warming of 1.5℃, and then 2℃, will be exceeded this century unless we make deep cuts to CO₂ and other greenhouse gas emissions in coming decades.

Climate change and its consequences are already being felt. Beyond 1.5℃, the situation is likely to rapidly deteriorate.

We are among the climate scientists who contributed to the latest IPCC report, including on the question of 1.5C℃ warming. Here, we go beyond the headlines to explain how the 1.5℃ rise is measured – and why maintaining the lowest global warming possible is what really matters.

woman carries fan and sack
The IPCC says Earth is likely to get close to, or reach, 1.5℃ warming by the early 2030s. Amr Nabil/AP

‘The most important goal’

Under the 2015 Paris Agreement, nations agreed to hold global warming to well below 2℃, and preferably limit it to 1.5℃, compared to pre-industrial levels.

The first global stocktake of that agreement will be held in 2023, to assess the world’s progress towards achieving its goals. That’s one of the reasons global warming levels are being so keenly watched right now.

The IPCC’s latest findings say 1.5℃ warming will be reached or exceeded in the early 2030s in all emissions scenarios considered – except the highest emissions scenario, for which the crossing could occur even earlier.

But not all hope is lost. In the very low emissions scenario considered in the report – known officially as “SSP1-1.9” – Earth reaches 1.5℃ warming for a few decades, but drops back below it by the end of the century.

This point is important. It’s still possible for Earth to keep below 1.5℃ global warming this century, if we rapidly cut emissions to net-zero. All other scenarios lead to further global warming once 1.5℃ is reached.

However, if maintaining 1.5℃ is not possible, the next goal should be to limit global warming to 1.6℃, then 1.7℃ and so on. Limiting warming to the lowest possible level is the most important goal. Every bit of warming we avoid will reduce the climate risks we face.


Read more: This is the most sobering report card yet on climate change and Earth’s future. Here’s what you need to know


wind turbines
It’s still possible for Earth to keep below 1.5℃ global warming this century, if we cut emissions quickly and deeply. Shutterstock

How 1.5℃ warming is measured

The declaration that Earth has reached 1.5℃ warming since the pre-industrial era will not be made after a single year, or a single location, passes that threshold.

The warming is measured as a global average over 20 years, to account for natural variability in the system.

Before global average temperatures officially reach 1.5℃ warming, we can expect quite a few years will exceed that limit. In fact, global temperatures exceeded 1.5℃ warming during individual months at the peak of the 2015-16 El Niño.

The industrial era – and associated greenhouse gas emissions – started in the 1700s. But there is almost no observed climate data on land outside Europe before the mid-19th century.

So, the period of 1850-1900 is used to approximate pre-industrial conditions. The IPCC estimates there was a likely temperature change of between -0.1 to +0.3℃ for the century or so before this where climate data is lacking.

According to the latest IPCC findings, Earth’s average temperature in the last decade was 1.09℃ warmer than the pre-industrial baseline. Obviously, this goes most of the way to 1.5℃ of warming. The IPCC says this warming is unequivocally the result of human influence.

two men lie in sun
Earth has warmed by 1.09℃ since pre-industrial times. Joel Carrett/AAP

The new science

Several innovations have helped inform the IPCC’s latest assessment. For the first time, the IPCC’s estimate of future global temperature change is based on three factors.

First are projections using new scenarios “Shared Socio-economic Pathways” or SSPs. Each pathway refers to different trajectories the world’s society and economy could take, and the emissions that would result.

A range of climate models – the result of much global scientific effort – is used to simulate climate change in response to each pathway.

Second, climate models are verified against observed climate data. Climate models are essential tools, but should always be used carefully. This grounding in observations was particularly needed with the latest round of climate models, to bring their results in line with other types of evidence.


Read more: Yes, a few climate models give unexpected predictions – but the technology remains a powerful tool


Third, the IPCC used an assessment of “climate sensititivty” – how sensitive Earth’s temperature is to a doubling of global CO₂ concentrations. The IPCC’s assessment of the evidence puts climate sensitivity at likely between 2.5℃ and 4℃, with low-likelihood possibilities of less than 2℃ or more than 5℃.

If humanity is lucky, and actual climate sensitivity is in the lowest plausible range, Earth may not reach the 1.5℃ warming limit under the lowest emissions scenarios (but still will under the medium or high ones). If we are unlucky and climate sensitivity is in the high range, the need to quickly reach net-zero emissions becomes even greater.

The following diagram from the latest IPCC report shows the estimated timeframe for reaching various global warming levels, under different Shared Socio-economic Pathways and climate sensitivity values.

IPCC

Where does this leave us?

The latest IPCC findings confirm Earth will be in the ballpark of 1.5℃ warming in the early 2030s. What happens after that depends on the decisions we make today.

With deep and sustained reductions in CO₂ and other greenhouse gas emissions in coming decades, we could keep warming around the 1.5℃ mark and then bring it below that threshold by the end of the century.

The IPCC findings are worrying, but should not be a distraction from our global climate efforts. Staying below 1.5℃ warming is important. But maintaining the lowest global warming we can – whether or not we exceed the 1.5℃ goal – is what really matters.

To explore climate change in your region and around the world at 1.5℃ and higher global warming levels, see the IPCC’s Interactive Atlas.

 

 

 


Read more: Climate change has already hit Australia. Unless we act now, a hotter, drier and more dangerous future awaits, IPCC warns


Click here to read more of The Conversation’s coverage of the IPCC reportThe Conversation

Michael Grose, Climate projections scientist, CSIRO; Malte Meinshausen, A/Prof., School of Earth Sciences, The University of Melbourne; Pep Canadell, Chief research scientist, Climate Science Centre, CSIRO Oceans and Atmosphere; and Executive Director, Global Carbon Project, CSIRO, and Zebedee Nicholls, PhD Researcher at the Climate & Energy College, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.