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.

MIT News Office

Just as an oven gives off more heat to the surrounding kitchen as its internal temperature rises, the Earth sheds more heat into space as its surface warms up. Since the 1950s, scientists have observed a surprisingly straightforward, linear relationship between the Earth's surface temperature and its outgoing heat.

But the Earth is an incredibly messy system, with many complicated, interacting parts that can affect this process. Scientists have thus found it difficult to explain why this relationship between surface temperature and outgoing heat is so simple and linear. Finding an explanation could help climate scientists model the effects of climate change.

Now scientists from MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS) have found the answer, along with a prediction for when this linear relationship will break down.

They observed that Earth emits heat to space from the planet's surface as well as from the atmosphere. As both heat up, say by the addition of carbon dioxide, the air holds more water vapor, which in turn acts to trap more heat in the atmosphere. 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 warmer atmosphere emits more heat into space.

The overall change in Earth's emitted heat thus only depends on the surface. In turn, the emission of heat from Earth's surface to space is a simple function of temperature, leading to to the observed linear relationship.

Their findings, which appear today in the Proceedings of the National Academy of Sciences, may also help to explain how extreme, hothouse climates in Earth's ancient past unfolded. 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

In their search for an explanation, the team built a radiation code — essentially, a model of the Earth and how it emits heat, or infrared radiation, into space. The code simulates the Earth as a vertical column, starting from the ground, up through the atmosphere, and finally into space. Koll can input a surface temperature into the column, and the code calculates the amount of radiation that escapes through the entire 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.

“So the radiation code gave us what Earth actually does," Koll says. “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."

To do this, the team 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.

“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 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.

In the end, the researchers observed that as the Earth's surface temperature gets hotter, it essentially wants to shed more heat into space. But at the same time, water vapor builds up, and acts to absorb and trap heat at certain wavelengths, creating 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," Koll says. “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 escape into space is directly related to the surface temperature, as the increase in heat emitted by the atmosphere is cancelled out by the increased absorption from water vapor.

Tipping towards Venus

The team found this linear relationship breaks down when Earth's global average surface temperatures go much beyond 300 K, or 80 F. In such a scenario, it would be much more difficult for the Earth to shed heat at roughly the same rate as its surface warms. For now, that number is hovering around 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 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.

“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 wouldn't kick in until 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, “at which point, we would turn into Venus."

Koll says 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," 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 research was funded, in part, by the National Science Foundation, and the James S. McDonnell Foundation.

Reprinted with permission of MIT News


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International poker champion Liv Boeree teaches decision-making for Big Think Edge.

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Five Hawks Down: watch the tragic migration of six Californian raptors

Tracking project establishes northern Argentina is wintering ground of Swainson's hawks

Image: @TrackingTalons / Ruland Kolen
popular
  • Watch these six dots move across the map and be moved yourself: this is a story about coming of age, discovery, hardship, death and survival.
  • Each dot is a tag attached to the talon of a Swainson's Hawk. We follow them on their very first migration, from northern California all the way down to Argentina.
  • After one year, only one is still alive.

Discovered: destination Argentina

Image: @TrackingTalons

Young Swainson's hawks were found to migrate to northern Argentina

The Buteo swainsoni is a slim, graceful hawk that nests from the Great Plains all the way to northern California.

It feeds mainly on insects, but will also prey on rodents, snakes and birds when raising their young. These learn to fly about 45 days after hatching but may remain with their parents until fall migration, building up flying skills and fat reserves.

A common sight in summer over the Prairies and the West, Swainson's hawks disappear every autumn. While it was assumed they migrated south, it was long unclear precisely where they went.

A group of researchers that has been studying raptors in northern California for over 40 years has now established exactly where young Swainson's hawks go in winter. The story of their odyssey, summarized in a 30-second clip (scroll down), is both amazing and shocking.

Harnessing the hawks

Image: @TrackingTalons, found here on imgur.

A Swainson's hawk, with tracking device.

The team harnessed six Swainson's hawks in July, as they were six weeks old and just learning to fly. The clip covers 14 months, until next August – so basically, the first year of flight.

Each harness contains a solar-powered tracker and weighs 20 grams, which represents just 3% of the bird's body weight. To minimize the burden, only females were harnessed: as with most raptors, Swainson's hawk females generally are bigger than males.

The first shock occurs just one month (or about 2.4 seconds) from the start of the clip: the first dot disappears. The first casualty. A fledgling no more than two months old, who never made it further than 20 miles from its nest.

By that time, the remaining five are well on their way, clustering around the U.S.-Mexico border in Texas. Swainson's hawks usually travel at around 40 mph (65 km/h) but can almost double that speed when they're stooping (i.e. dive down, especially when attacking prey).

'Migration unrest'

First year of life for six Swainson's Hawks [OC] from r/dataisbeautiful

There's a strong genetic component to migration. As usual, the Germans have nice single word to summarize this complex concept: Zugunruhe ('tsook-n-roowa'), literally: 'migration unrest' (1). It denotes the seasonal urge of migratory animals – especially birds – to get on their way. Zugunruhe exhibits especially as restless behavior around nightfall. The number of nights on which it occurs is apparently higher if the distance to be traveled is longer.

The birds may have the urge to go south, but genetics doesn't tell them the exact route. They have to find that out by trial and error. Hence the circling about by the specimens in this clip: they're getting a sense of where to find food and which direction to go. Their migratory paths will be refined by experience – if they're lucky enough to survive that long.

Each bird flies solo: their paths often strongly diverge, and if they seem to meet up occasionally, that's just an illusion: even when the dots are close together, they can still be dozens if not hundreds of miles apart.

Panama snack stop

Image: @TrackingTalons

The Central American isthmus is a major bird migration corridor

They generally follow the same route as it is the path of least resistance: follow mountain ranges, stay over land. Like most raptors, Swainson's hawks migration paths are land-based: not just so they can roost at night, but mainly to benefit from the thermals and updrafts to keep them aloft. That reduces the need to flap wings, and thus their energy spend – even though the trip will take longer that way.

As this clip demonstrates, the land-migration imperative means the Central American isthmus is a hotspot for bird migration. Indeed, Panama and Costa Rica are favorite destinations for bird watchers, when the season's right. A bit to the north, Veracruz in Mexico is another bird migration hotspot.

It's thought most hawks don't eat at all on migration. This clip shows an exception to that rule: on the way back, one bird takes an extended stopover of a couple of weeks in Panama, probably spending its time there foraging for food.

So, when they finally arrive in northern Argentina, after 6 to 8 weeks' migration, the hawks are pretty famished. Until a few decades ago, they fed on locusts. For their own reasons, local farmers have been getting rid of those. The hawks now concentrate on grasshoppers, and basically anything else that's edible.

For first-time visitors, finding what they need is not easy. Three of the five dots go dark. These birds probably died from starvation. But two birds thrive: they roam the region until winter rears its head in South America, and it's time to head back north again, where summer is getting under way.

Both dots make it back across the border, but unfortunately, right at the end of the clip, one of the surviving two birds expires.

Harsh, but not unusual

Image: @TrackingTalons, found here on imgur.

This old lady is 27 years old, but still nesting.

While a one-in-six survival rate may seem alarmingly harsh, it's not that unusual. First-year mortality for Swainson's Hawks is between 50% and 80%. Disease, starvation, predators and power lines – to name just a few common causes of death - take out a big number.

Only 10% to 15% of the young 'uns make it past their third or fourth year into adulthood, but from then on, annual survival rates are much better: around 90%. Adult Swainson's Hawks can expect to live into their low teens. There's one documented example of a female Swainson's Hawk in the wild who was at least 27 years old (and still nesting!)

The Californian population of Swainson's Hawks plummeted by about 90% at the end of last century but is now again increasing well. The monitoring project that produced this clip has been going for about four decades but is seeing its funding dry up. Check them out and consider supporting them (see details below).

Image: Yale School of Forestry and Environmental Studies

Migration trajectory of B95, the 'Moonbird'.

Not all migrating birds shun the ocean. Here's an incredible map of an incredible migration path that's even longer than that of the Swainson's hawks.

In February 1995, a red knot (Calidris canutus rufa) in Tierra del Fuego (southern Argentina) was banded with the tag B95. That particular bird, likely born in 1993, was recaptured at least three times and resighted as recently as May 2014, in the Canadian Arctic.

B95 is more commonly known as 'Moonbird', because the length of its annual migration (app. 20,000 miles; 32,000 km) combined with its extreme longevity (if still alive, it's 25-26 years old now) means its total lifetime flight exceeds the distance from the Earth to the Moon.

As many other shorebirds do, the red knot takes the Atlantic Flyway hugging the coastline and crossing to South America via the ocean.

B95 has become the poster bird of conservationists in both North and South America. A book titled Moonbird: A Year on the Wind with the Great Survivor B95 (2012) received numerous awards, B95 has a statue in Mispillion Harbor on Delaware Bay and the City of Rio Grande on Tierra del Fuego has proclaimed B95 its natural ambassador.

Perhaps one day the nameless Swainson's Hawks in this clip, fallen in service of their ancestral instincts – against the odds of human increasing interference – will receive a similar honor.

Migration clip found here at the DataIsBeautiful subreddit. Read through the comments to learn a lot more about Swainson's Hawks, and raptors in general.

Check out the California raptor tracking programme 'Tracking Talons' on Twitter at @TrackingTalons, on their Facebook page, and on their website.

Strange Maps #965

Got a strange map? Let me know at strangemaps@gmail.com.

(1) 'Zug' is a wonderfully polyvalent German word. It can mean: a train, a chess move, a characteristic, a stroke, a draft (of a plan), a gulp (of air), a drag (from a cigarette), a swig (from a bottle), and more.

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An Atlantic horseshoe crab in an aquarium. Photo: Domdomegg via Wikimedia Commons.
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