A warming Earth might eventually copy the greenhouse effect of Venus


Modelling finds the precious equilibrium between temperature and radiation breaks down beyond a certain point, spelling big trouble. Alan Duffy reports.


Surface temperature and the loss of heat to space are finely balanced.

Surface temperature and the loss of heat to space are finely balanced.

MIT

Climate models show a warming Earth radiates more heat into space, but when pushed too far, and this release valve shuts down, temperatures skyrocket to potentially Venusian levels.

Anyone familiar with a cooker knows that the hotter it gets the more heat it radiates. So too with the Earth. Remarkably for such a complex system, however, the heat radiating from the planet is in linear relation with the temperature increase of its surface – a mysteriously simple arrangement known to scientists since the 1950s.

Now, climate models published in the journal Proceedings of the National Academy of Science, calculated by researchers at the Massachusetts Institute of Technology’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), have revealed why this linear relation holds – and when it may break down to the detriment of all life.

The EAPS team, led by Tim Cronin, modelled the Earth in a vertical column from the surface through the atmosphere and into space, tracking the progress of heat, or infrared radiation. Increasing the surface temperature in the model led to a similar increase in the rate of escaping heat from the atmosphere.

This appeared to hold true to temperatures over 60 degrees Kelvin (-213 °C). EAPS postdoc Daniel Koll then explored the code “to see which of these physics is actually responsible for this relationship”.

The key step in the model was when the infrared radiation was broken into 350,000 separate spectral intervals, which allowed the team to capture a strong interaction with water at an incredibly specific frequency.

The water vapour in the atmosphere absorbs and then re-radiates infrared radiation from the warming Earth’s surface. According to the model, this atmospheric effect meant that increasing heat loss from the air was balanced by increasing absorption from the water vapour.

However, not all of the heat is emitted by the water vapour, limiting in turn how much heat can be emitted from the surface. This is known as the greenhouse effect.

“It's like there's a window, through which a river of radiation can flow to space,” Koll explains.

“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.”

Confident in their model, the EAPS team then cranked up the surface to ever hotter temperatures, finding that the simple linear relation broke down at 300 °K (27 °C). Above this threshold, the increasing temperature of the surface didn’t lead to a similar increase in the amount of heat escaping, thus trapping more heat.

The rising surface temperature led to more water in the atmosphere, which in turn trapped the heat, causing ever higher temperatures, resulting in a runaway greenhouse effect. This process is believed to have occurred on Earth’s twin, the planet Venus.

“Some time in the past, we think its atmosphere had a lot of water vapour, and the greenhouse effect would've become so strong that this window region closed off, and nothing could get out any more, and then you get runaway heating,” Koll says.

Koll calculated that for Earth the runaway temperature threshold was at 340 °K (67 °C) – thankfully far beyond the current average surface temperature of about 285 °K (12 °C).

Such dramatic temperature rises could only be possible through cataclysmic events, such as increasing solar outputs over billions of years of the sun’s evolution. The EAPS work suggests that the more modest, yet still dangerous, climate change-inducing temperature rises the existing model predicts for Earth will see the linear relation hold true.

So, while Earth won’t go the way of Venus any time soon, a warming planet still means a hotter world despite the increasing heat escaping into space.

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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne, and Lead Scientist of The Royal Institution of Australia. Twitter | @astroduff
  1. http://www.pnas.org/content/early/2018/09/24/1809868115
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