freediver
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This is not just a theory, it is based on “observational evidence”, that is, scientists have data that shows the increased carbon dioxide in the atmosphere is coming from fossil fuels and land clearing.
AND NIGHTS ARE GETTING HOTTER
We can also look at other observational data to help strengthen the theory.
If the Earth was warming up because of increasing sunlight, then you would expect temperatures during the day to increase and for it to be cooler at night (because there is no sun at night!).
However, what scientists found is that nights were actually warming up more so than days.
This points to greenhouse gases playing a role.
As noted above, greenhouse gases trap heat radiation from the Earth and stop it from being released into space as quickly.
This effect can be seen for example, on nights with more clouds, which don’t cool down as much as there is more water vapour in the atmosphere.
In contrast, deserts are more cool at night because there is not as much water vapour over these areas, and it’s a similar story in coastal areas.
So if nights are warming up more than days, it’s unlikely that the sun is playing a role in this, it’s more likely that greenhouse gases are trapping heat on Earth and pushing up temperatures.
Scientists have also looked at temperatures in the Earth’s stratosphere, which is the layer of the atmosphere from about 10km up.
The stratosphere warms because the ozone layer it contains absorbs the sun’s ultraviolent radiation.
If there was more sunlight, you would expect the upper atmosphere to warm up because it was absorbing more ultraviolet rays.
But if there was an increase in greenhouse gases then you would expect the stratosphere to be cooler because carbon dioxide is efficient, not only at absorbing heat radiation but also at releasing it into space, cooling it down.
“Observations have shown that the surface and lower atmosphere have warmed, and the upper atmosphere has cooled in the last 50 years — the entire time we’ve been monitoring it through balloons and other satellites,” Prof Karoly said.
“This pattern of temperature change has happened everywhere and cannot be explained by increasing sunlight,” he said. “And it’s been getting stronger, which is exactly what you would expect from increasing greenhouse gases in the atmosphere.”
NOW ADD COMPUTERS TO THE EQUATION
The first approach to looking at climate change is “observational data” but you can also use complex mathematical models of the climate system.
Around the world, Prof Karoly said more than 50 complex climate models had been developed to test climate theories on a larger scale.
While some may question how scientists could simulate the climate when they can’t forecast the weather over long periods of time, Prof Karoly said it was because the climate models looked at levels of radiation, which determine long-term climate.
“Models solve physical equations for the absorption and transmission of radiation in the atmosphere, and for the motion of the air, and for the motions of the ocean,” he said.
These simulations have shown that without human influences there would not be any long-term warming trend.
Temperatures would have stayed pretty much the same with only two-tenths of a degree of warming.
Instead the world has warmed by 1.1 degrees and the warming over Australia has been even higher than the global average, at 1.5 degrees.
This is because land warms up faster than the ocean.
WHAT ABOUT THE BUSHFIRES?
So how does this relate to the catastrophic bushfires that have raged across Australia in recent months?
Higher mean temperatures give rise to a greater chance of heatwaves and hot extremes, Prof Karoly said.
“We have good observational data of the current summer and the last 50 years,” he said.
“There have been marked increases in heatwaves and hot days in all parts of Australia.”
Australia experienced its hottest and driest year on record in 2019 and December 2019 had a number of Australia’s hottest days ever recorded.
“We have also seen increases in sea levels, exactly what you would expect from climate change and the warming of ocean waters and melting of ice sheets and glaciers on land.”
When it comes to the intensity of bushfires, Prof Karoly said there are certain factors that were known to be important.
The McArthur Forest Fire Danger Index was developed to measure the degree of fire danger in Australian forests and the likelihood they will occur.
It combines factors including the temperature of air, wind speed, the dryness of the air (measured by relative humidity) and the dryness of the fuel and the ground (measured by rainfall over the previous month).
“So the combination of high temperatures, strong winds, low humidity and no rainfall leads to extreme fire danger,” Prof Karoly said.
These were exactly the conditions experienced in NSW and southern Queensland in September and October where there were record high temperatures and low humidity.
These conditions were also experienced in Canberra, coastal NSW and particularly East Gippsland in Victoria, which was why there was extreme fire danger in these areas.
The next question is whether climate change caused these conditions.
Prof Karoly says climate change has led to higher temperatures, as discussed above, but it’s unlikely it had a major role in the drought conditions.
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