The Climate System Lab

The Climate System Lab

Why is the equator drenched in rainforest while deserts ring the planet at 30 degrees? Start with an imbalance. At the equator the sun is nearly overhead, so its energy lands on a small patch of ground. Near the poles the same beam strikes at a shallow angle and is smeared across a much larger area, having travelled further through the atmosphere on the way. The tropics take in far more energy than they lose; the poles lose far more than they take in. If nothing moved, the tropics would go on heating and the poles go on cooling for ever. They do not, because the atmosphere and the oceans carry the surplus polewards. Everything in this module is a consequence of that one transfer.

Rising air and sinking air

Almost all of it comes down to whether air at a given latitude is going up or coming down:

Label the circulation

This diagram shows one part of global atmospheric circulation. Drag each label to the correct place.

Why does rising air cool?

Rising air cools, which is what makes it rain. But nothing is taking heat out of it. So why does it cool?

• Air pressure falls with height, so the rising air expands, and expanding uses up its own energy, which lowers its temperature
• It moves further away from the warm ground that was heating it
• It mixes with cold air coming down from the poles
• It radiates its heat away into space

One Hadley cell

Put the stages of a Hadley cell into order, starting at the equator.

• The sun heats the surface most strongly at the equator
• The warm air rises, giving low pressure and heavy rain
• High above the surface, the now dry air flows away towards the poles
• It cools, becomes denser and sinks at about 30 degrees, giving high pressure
• The dry surface air flows back towards the equator, and the loop closes

The rest of the system

The Hadley cell is one of three, and the atmosphere is not doing the job alone:

Two places, one latitude

Plymouth in Devon and Goose Bay in Labrador, Canada, sit at almost exactly the same latitude. Plymouth rarely drops below freezing; Goose Bay averages around minus 17 Celsius in January. Why?

• Warm water is carried north-east across the Atlantic towards Europe, and the prevailing westerly winds blow across it before reaching Britain, arriving mild and moist
• Plymouth receives noticeably more sunlight than Goose Bay
• Goose Bay is at a much higher altitude
• Goose Bay lies in the Polar cell and Plymouth lies in the Ferrel cell

Natural climate change

Climate changed long before anybody was burning anything. Three natural causes: asteroid impacts, which loft dust that blocks sunlight; orbital variation, the slow Milankovitch wobbles in the shape of Earth's orbit and the tilt of its axis, which pace the ice ages; and volcanism, which throws ash and sulfate gases high enough to shade the planet. The last of those has a documented case. Mount Tambora, in Indonesia, erupted in April 1815, the largest eruption in recorded history. Its sulfate haze spread worldwide, and 1816 became known across Europe and North America as the year without a summer: snow fell in June in New England, harvests failed, and food prices climbed steeply. We know about earlier climates from ice cores, tree rings and historical sources.

What does each source record?

How far back?

Put the three sources of evidence in order of how far back they can take us, shortest reach first.

• Historical sources: hundreds of years, and only where somebody was writing things down
• Tree rings: thousands of years, by overlapping living trees with older dead timber
• Ice cores: hundreds of thousands of years, through several complete ice ages

Human-caused change

Recent warming is rapid and is mainly ours. Burning fossil fuels, clearing forests and farming livestock release carbon dioxide and methane. How that traps heat. Sunlight arrives as short-wave radiation and passes through the atmosphere almost untouched. The ground absorbs it and re-emits it as long-wave infrared, and it is that outgoing long-wave that carbon dioxide and methane absorb, re-emitting it in every direction including back down. The gases are not a blanket over the planet. They are selectively transparent: open on the way in, partly closed on the way out. The evidence is rising carbon dioxide, rising global temperature, rising sea level and retreating glaciers. Why projections for 2100 differ so much. Chiefly because they depend on choices nobody has made yet: how much is emitted between now and then. Feedback loops add to the range, since melting ice exposes dark ocean that absorbs more heat, which melts more ice.

Natural or human?

Each of these is a real piece of evidence. Select the THREE that point to a NATURAL cause.

• A thin layer of volcanic ash in an ice core, directly above a band showing two unusually cold years
• Ice ages arriving in a repeating rhythm of roughly a hundred thousand years, stretching back long before any human civilisation
• A worldwide dust layer, rich in metals that are rare on Earth but common in meteorites, at the boundary where the dinosaurs disappear
• Atmospheric carbon dioxide climbing steadily and without a break from about 1850 onwards
• Methane concentrations rising alongside a rapid growth in the number of cattle farmed worldwide
• Satellite images showing a steady loss of tropical forest cover decade on decade

What the range means

Projections for global temperature in 2100 span roughly 3 degrees between the lowest and the highest. A student argues that such a wide range proves climate science is too unreliable to act on. What is the best response?

• The range is mostly not disagreement about the physics. It is the difference between emissions pathways, so the width of it reflects choices still to be made rather than ignorance about how the atmosphere works
• They are right. A 3 degree spread means the models cannot be trusted
• Scientists disagree about whether carbon dioxide warms the planet, and the range shows that disagreement
• The range is simply measurement error, and better thermometers would narrow it

Climate summary

The tropics absorb more energy than they lose and the poles lose more than they absorb, so the atmosphere and oceans carry the surplus _____. At the equator air rises and cools, giving low pressure and rain; at about 30 degrees it sinks, giving _____ pressure and hot deserts, and that loop is the _____ cell. Greenhouse gases warm the surface because they let short-wave sunlight in but absorb the _____ radiation the ground sends back out.