Climate models

How to use climate models


The new report from the Australian Academy of Sciences, The risks for Australia of a 3 ° C hotter world, has grim predictions for the nation’s future under current carbon emissions policy and action. All of these predictions are based on climate models.

Among the now familiar predictions of ecological and economic damage, the report points out that Alice Springs could see a 213% increase in energy demand by the turn of the century (page 57), Hobart could see an increase of 45 % of Ross River virus cases by 2079 (p 64), and one in 19 homeowners could face unaffordable insurance premiums by 2030 (p 59).

How do climatologists arrive at numbers like this? What does climate modeling consist of? How are climate models applied?

Unsurprisingly, it varies wildly with what you’re trying to predict. But there are a few key things researchers need to predict the effects of climate change.

Expertise in different fields

Climate forecasting requires expertise from a range of different scientific and economic fields.

“The more connections you have and the wider the range of perspectives involved, the more robust your models are,” says Ove Hoegh-Guldberg, chair of the report and professor of marine studies at the University of Queensland.

A researcher may have a deep understanding of how warmer temperatures affect certain crops, but it takes a different area of ​​study – and therefore a different researcher – to understand how their yields can differ on a large scale.

Hoegh-Guldberg’s work on coral reefs in the 1980s suggested they were vulnerable to warmer ocean temperatures, but this required the work of scientists and global systems modelers to reveal the extent of the risk to the reefs. corals.

“When it comes to global models, I’m a user, not a builder,” he says.

Predict the climate

Before we understand how we will be affected, we must understand how the climate will change. And that can be difficult to do with precision.

“The ability to predict future climate change is a fine art,” says Hoegh-Guldberg.

“There are uncertainties in terms of emissions and this depends on population, technology, policies and consumption patterns,” said Mark Howden, member of the report’s expert group and director of the Institute. for Climate, Energy and Disasters from the Australian National University.

“For any type of level of greenhouse gas emissions, there is scientific uncertainty as to how this will translate into increases in temperature. And then there are uncertainties about how we will adjust to these increases in, say, health outcomes. “

This is why predictions vary so much, and why they will often be reported with large error bars.

That said, climate models are tested against past events and may prove to be more accurate.

“In many cases, it can be helpful to see how well the models can perform retrospective simulation,” says Hoegh-Guldberg. “Essentially, going back in time to see how well your model explained what really happened. “

If the models are able to accurately guess how the Earth reacted to volcanic eruptions, for example, they are likely to be effective in predicting the increase in temperature over the next century.

There are a myriad of climate models and simulations that can be used to predict changes in temperature. The more we use, the better: several simulations can produce a more precise result between them.

One popular tool is the General Circulation Models, or GCMs. The CoastAdapt website, which can predict sea level rise and temperature for individual communal areas under different emission scenarios, uses these models. He points out that these are not precise predictions and that a number of models should be used to explore plausible futures for your suburb.

Human activity must also be taken into account. A simple example is energy use – for example, if Australian cities experience more heat waves, they will also use more energy and produce more emissions to mitigate those heat waves. This is just one of many more complicated feedback loops.

Predict human outcomes with models

It can be even more complicated to use these models to predict the economic and health effects on people, but a lot of resources are invested in this area as well. Insurance companies, governments and businesses have a vested interest in knowing what will happen in their region over the next several decades.

“We basically create devices that allow us to make decisions and operate complex systems,” says Hoegh-Guldberg.

Economic costs are evaluated using complex computer models, using various software packages. Some common models used to predict climate are computable general equilibrium models, or CGEs, and integrated assessment models, or IAMs. EGCs provide detailed pictures of the economies of countries and regions, while MOIs focus more on the interactions between the economy and the environment and how they influence each other.

There are a few other methods of assigning a price to losses due to climate change. One estimate might assess the value of all infrastructure in a certain area, while another might look at the cost of an insurance payment if the area were affected by a natural disaster.

Likewise, health outcomes can be assessed in a number of ways – from predicting the incidence of a particular disease to changes in life expectancy in the general population. Again, it needs medical specialists, public health experts, and climate modelers working together to make predictions.

A thoughtful way to report predictions

It’s one thing to make predictions about the climate future, and it’s another to report them. Uncertainty, in particular, can be a very difficult thing to communicate.

In scientific reports, predictions will be listed with error bars. For example, one study (described on page 53 of the report) suggests that with a warming of 3 ° C, by 2090, Darwin could experience 180 to 322 days per year with temperatures above 35 ° C, with a average estimate of 265 days.

So how do you tell people what you have found? Do you lead with the (relatively) optimistic 180 days, the worst case 322 days, or the most likely value of 265?

“I think of all of the above,” says Hoegh-Guldberg.

“Understanding the range of the number of days affected provides important information, as does the average. “

These gloomy predictions are very carefully developed, but there will always be inaccuracies. It may be better to focus on events that have already happened to gauge the severity of the climate crisis.

Understanding the current losses and disasters, like the Black Summer bushfires, is enough to realize how quickly emissions need to go down, Howden says.

“The urgency of the situation, I think, is very obvious to anyone who wants to look at what’s already going on,” he said. “Then we can draw the dots to the future. “

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