The Earth sheds more heat into space as its surface heats up, in the same way as an oven radiates more heat to the surrounding kitchen as its internal temperature increases. Since the 1950s, researchers have witnessed a remarkably straightforward, linear relationship between the Earth’s surface temperature and its outgoing heat.
But the Earth is an extremely messy system, with complex, interacting parts that can impact this process. Researchers have thus found it hard to explain why this relationship between surface temperature and outgoing heat is so linear and simple. Obtaining an explanation could help climate scientists to model the effects of climate change.
Currently, researchers from MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) have found the answer, together with a prediction for when this linear relationship will collapse.
They noticed that Earth discharges heat to space from both the planet’s surface and from the atmosphere. As both heat up, say because of the addition of carbon dioxide, the air holds more water vapor, which, in turn, serves to capture more heat in the atmosphere. This strengthening of Earth’s greenhouse effect is called water vapor feedback. Significantly, the team found that the water vapor feedback is just adequate to cancel out the rate at which the warmer atmosphere releases more heat into space.
The total change in Earth’s discharged heat thus only relies on the surface. In turn, the emission of heat from Earth’s surface to space is a basic function of temperature, resulting in the observed linear relationship.
Their findings, which have been reported in the September 24th issue of the Proceedings of the National Academy of Sciences, may also help to clarify how extreme, hothouse climates in Earth’s ancient past came to be. The paper’s co-authors are EAPS postdoc Daniel Koll and Tim Cronin, the Kerr-McGee Career Development Assistant Professor in EAPS.
A window for heat
Keen on finding an explanation, the team developed a radiation code — fundamentally, a model of the Earth and how it discharges heat, or infrared radiation, into space. The code simulates the Earth as a vertical column, beginning from the ground, up through the atmosphere, and lastly into space. Koll can input a surface temperature into the column, and the code calculates the amount of radiation that discharges through the whole column and into space.
The team can then alter the temperature knob up and down to see how various surface temperatures would impact the outgoing heat. When they plotted their data, they noticed a straight line — a linear relationship between surface temperature and outgoing heat, in line with many earlier works, and over a range of 60 K, or 108 °F.
So the radiation code gave us what Earth actually does. Then I started digging into this code, which is a lump of physics smashed together, to see which of these physics is actually responsible for this relationship.
Daniel Koll, Postdoc, EAPS
To achieve this, the team programed into their code different effects in the atmosphere, such as convection, and humidity, or water vapor, and rotated these knobs up and down to understand how they in turn would influence the Earth’s outgoing infrared radiation.
“We needed to break up the whole spectrum of infrared radiation into about 350,000 spectral intervals, because not all infrared is equal,” Koll says.
He explains that, while water vapor does capture heat, or infrared radiation, it does not absorb it extensively, but at wavelengths that are extremely specific, so much so that the team had to split the infrared spectrum into 350,000 wavelengths just to detect precisely which wavelengths were absorbed by water vapor.
In the end, the scientists witnessed that as the Earth’s surface temperature gets hotter, it fundamentally wants to emit more heat into space. But simultaneously, water vapor builds up, and acts to absorb and trap heat at specific wavelengths, causing a greenhouse effect that prevents a fraction of heat from escaping.
It’s like there’s a window, through which a river of radiation can flow to space. The river flows faster and faster as you make things hotter, but the window gets smaller, because the greenhouse effect is trapping a lot of that radiation and preventing it from escaping.
Koll says this greenhouse effect explains why the heat that does discharge into space is directly connected to the surface temperature, as the increase in heat discharged by the atmosphere is negated by the increased absorption from water vapor.
Tipping towards Venus
The team learned that this linear relationship breaks down when Earth’s universal average surface temperatures go much beyond 300 K, or 80 °F. In such a situation, it would be much harder for the Earth to shed heat at approximately the same rate as its surface warms. For now, that number is hovering about 285 K, or 53 °F.
“It means we’re still good now, but if the Earth becomes much hotter, then we could be in for a nonlinear world, where stuff could get much more complicated,” Koll says.
To give an idea of what such a nonlinear domain might look like, he invokes Venus—a planet that many researchers believe began as a world similar to Earth, though a lot closer to the sun.
“Some time in the past, we think its atmosphere had a lot of water vapor, and the greenhouse effect would’ve become so strong that this window region closed off, and nothing could get out anymore, and then you get runaway heating,” Koll says.
In which case the whole planet gets so hot that oceans start to boil off, nasty things start to happen, and you transform from an Earth-like world to what Venus is today.
For Earth, Koll calculates that such a runaway effect would not occur until universal average temperatures touch about 340 K, or 152 °F. Global warming alone is inadequate to cause such warming, but other climatic variations, such as Earth’s warming over billions of years because of the sun’s natural evolution, could push Earth towards this limit, “at which point, we would turn into Venus.”
Koll says the team’s results may help to enhance climate model predictions. They also may be valuable in understanding how ancient hot climates on Earth came to be.
“If you were living on Earth 60 million years ago, it was a much hotter, wacky world, with no ice at the pole caps, and palm trees and crocodiles in what’s now Wyoming,” Koll says. “One of the things we show is, once you push to really hot climates like that, which we know happened in the past, things get much more complicated.”
This study was sponsored, partly, by the National Science Foundation, and the James S. McDonnell Foundation.