How Earth sheds heat into space

New insights into the role of water vapor may help researchers predict how the planet will respond to warming.

Since the 1950s, scientists have observed a surprisingly straightforward, linear relationship between the Earth’s surface temperature and its outgoing heat.
Since the 1950s, scientists have observed a surprisingly straightforward, linear relationship between the Earth’s surface temperature and its outgoing heat.

The Earth sheds more heat into space as its surface heat up. In 1950, scientists observed a straightforward clear, linear connection between the Earth’s surface temperature and its active heat.

As the earth is extremely messy system, with many complicated, interacting parts that can affect this process, it is difficult for scientists to explain why this relationship between surface temperature and outgoing heat is so simple and linear.

Now, MIT scientists have discovered the appropriate response, alongside a forecast for when this linear relationship will break down. They saw that Earth emanates heat to space from the planet’s surface and additionally from the atmosphere. As both heat up, say by the expansion of carbon dioxide, the air holds more water vapor, which thusly acts to trap more heat in the air.

This strengthening of Earth’s greenhouse effect is known as water vapor feedback. Crucially, the team found that the water vapor feedback is just sufficient to cancel out the rate at which the hot atmosphere emits more heat into space.

The general change in Earth’s produced heat therefore just relies upon the surface. Thus, the discharge of heat from Earth’s surface to space is a simple function of temperature, prompting to the observed linear relationship.

The study in other words may also help to explain how extreme, hothouse climates in Earth’s ancient past unfolded.

During the study, scientists constructed a radiation code — basically, a model of the Earth and how it transmits heat, or infrared radiation, into space. The code reproduces the Earth as a vertical column, beginning from the ground, through the environment, lastly into space. Scientists can input a surface temperature into the section, and the code figures the measure of radiation that breaks through the whole column and into space.

The team can then turn the temperature knob up and down to see how different surface temperatures would affect the outgoing heat. When they plotted their data, they observed a straight line — a linear relationship between surface temperature and outgoing heat, in line with many previous works, and over a range of 60 kelvins, or 108 degrees Fahrenheit.

Daniel Koll, MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) said, “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.”

For this, scientists programmed into their code various effects in the atmosphere, such as convection, and humidity, or water vapor, and turned these knobs up and down to see how they in turn would affect the Earth’s outgoing infrared radiation.

Koll said, “We needed to break up the whole spectrum of infrared radiation into about 350,000 spectral intervals because not all infrared is equal.”

“While water vapor does absorb heat or infrared radiation, it doesn’t absorb it indiscriminately, but at wavelengths that are incredibly specific, so much so that the team had to split the infrared spectrum into 350,000 wavelengths just to see exactly which wavelengths were absorbed by water vapor.”

Scientists observed that as the Earth’s surface temperature gets hotter, it basically needs to shed more heat into space. And yet, water vapor develops and acts to ingest and trap heat at specific wavelengths, making a greenhouse impact that keeps a small amount of heat from getting away.

Koll said, “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.”

“This greenhouse effect explains why the heat that does escape into space is directly related to the surface temperature, as the increase in heat emitted by the atmosphere is canceled out by the increased absorption from water vapor.”

Scientists discovered this linear relationship separates when Earth’s global normal surface temperatures go much past 300 K, or 80 F. In such a situation, it would be significantly more troublesome for the Earth to shed heat at a generally indistinguishable rate from its surface heat. For the present, that number is floating around 285 K, or 53 F.

To give an idea of what such a nonlinear world might look like, he invokes Venus — a planet that many scientists believe started out as a world similar to Earth, though much closer to the sun.

Koll said, “Sometime 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. 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.”

Estimating such runaway effect for earth, scientists found that it would not affect earth until the global average temperatures reach about 340 K, or 152 F. Global warming alone is insufficient to cause such warming, but other climatic changes, such as Earth’s warming over billions of years due to the sun’s natural evolution, could push Earth towards this limit.

Koll said, “the team’s results may help to improve climate model predictions. They also may be useful in understanding how ancient hot climates on Earth unfolded.”

“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. 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.”

The study is published in the Proceedings of the National Academy of Sciences. Research team involves EAPS postdoc Daniel Koll and Tim Cronin, the Kerr-McGee Career Development Assistant Professor in EAPS.