Earth's climate can change with or without our contribution. That said, dramatic forces are needed to drive those changes.
Scientists at Duke University, working with NASA's Jet Propulsion Laboratory, have conducted a study published in the Journal of Climate adding new evidence and confirming what we already understand about the natural drivers of climate change.
"The study is all about what the climate system does when it's not being pushed by external forces," says Patrick Brown, a PhD student at Duke University's Nicholas School of the Environment. "Changes in the sun or changes in the earth's orbit or changes in the amount of greenhouse gases that have to do with something totally natural, those all will force the climate into various states, but it's still a natural change."
He and his team used global climate models and data from NASA satellite observation to better understand what the climate would do if there were no outside influences.
"When you aren't changing greenhouse gases in the atmosphere, when you aren't changing the brightness of the sun, when you aren't changing other things like the amount of aerosols in the atmosphere, the climate system does remain stable. If you aren't pushing it in one direction or another, it shouldn't move dramatically, and therefore the warming that we've seen over the past hundred years does have to be attributed to something like increases in greenhouse gases."
Earth systems also go through feedback loops. To give an example, the white surfaces of polar icecaps reflect solar energy into space. This is called albedo. If the global temperature goes up, we lose a percentage of the icecaps, lowering the earth's albedo. The earth absorbs more sunlight and gets warmer; melting more of the polar icecaps and so-on.
"If we were to double CO2, for example, in the atmosphere and there were no feedbacks working, we would get about one-third the warming that we're actually expecting," says Brown. "So these positive feedbacks do amplify human-caused global warming."
So, if we have these feedbacks, why doesn't that throw the climate into a natural state of global warming or cooling?
"One thing is that when the earth kind of gets naturally warmer, it's able to move a bunch of energy to locations on the planet where there aren't these positive feedbacks working. Another thing it does is that that it allows the earth to release a bunch of energy to space and then bring the temperature back down to where it was.
The bottom line would be that the earth is able to cool itself down after a natural, unforced warming event. And so what that means is that you really do need these external drivers like changes in greenhouse gases in order to drive the climate system into a different state. And so that does kind of undermine the popular idea that the climate system can just randomly move in and out of various states for no particular reason.
According to Brown, there currently is nothing natural forcing climate change. There is, however, a surplus of human-produced carbon dioxide. But he says this knowledge does aid in the predictive power of science.
"In the future with the given amount of greenhouse gases increases that we're expecting, we can then forecast or project how much warming we should expect because the climate system does react to these drivers," says Brown. "It's not just doing its own thing randomly without any rhyme or reason."