The Greenhouse Effect, the Enhanced Greenhouse Effect and Climate Feedbacks

The natural greenhouse effect. Incoming solar radiation is mostly short-wave. Greenhouse gases are almost transparent to it, so it passes through the atmosphere and is absorbed by the surface. The warmed surface then re-radiates energy as long-wave infrared. Greenhouse gases absorb long-wave radiation strongly and re-emit it in all directions, including downwards, so the lower atmosphere is warmed. The entire mechanism rests on that asymmetry: short-wave in, long-wave obstructed on the way out. Without it, the mean surface temperature would be about −18 °C instead of about 15 °C, so the natural greenhouse effect is what makes the planet habitable.

The enhanced greenhouse effect. Human activity adds greenhouse gases faster than sinks remove them. Carbon dioxide comes from burning fossil fuels, deforestation and cement manufacture; methane from ruminant livestock, flooded rice fields, landfill and leaks from gas systems; nitrous oxide mainly from nitrogen fertilisers; and halocarbons from industry. The result is a stronger absorption of outgoing long-wave radiation, so the planet warms until outgoing energy again balances incoming energy at a higher temperature.

Global warming potential and residence time. Global warming potential measures the warming caused by one kilogram of a gas relative to one kilogram of carbon dioxide over a stated period, usually one hundred years. On that basis methane is roughly 28 times as powerful and nitrous oxide roughly 265 times. Carbon dioxide nonetheless dominates because warming depends on potency multiplied by quantity, and carbon dioxide is emitted in vastly larger amounts and persists for centuries. Methane's residence time of about twelve years has a practical consequence: cutting methane produces measurable results within a couple of decades, whereas cutting carbon dioxide is what determines the long-run outcome.

Positive feedbacks amplify. The ice–albedo feedback is the standard example: warming melts ice, the darker ocean or ground exposed has a much lower albedo, so more solar radiation is absorbed, so warming increases and more ice melts. Thawing permafrost releases methane and carbon dioxide from previously frozen organic matter. Warming increases evaporation, and water vapour is itself a greenhouse gas. Warmer sea water dissolves less carbon dioxide, so a sink weakens. Forest dieback and peat fires convert stores into sources. In every case the output feeds back to reinforce the original change.

Negative feedbacks damp, but only partly. Higher carbon dioxide concentrations can raise rates of photosynthesis, and increased evaporation can thicken low cloud, which reflects incoming radiation. Both are real, but both are limited: the fertilisation effect is constrained by nitrogen, water and temperature, and cloud is ambiguous in sign because low cloud reflects while high cloud traps outgoing long-wave radiation. The honest summary is that negative feedbacks exist but are not currently strong enough to offset the positive ones.

Tipping points. A tipping point is a threshold beyond which change becomes self-sustaining and is not reversible on human timescales, because positive feedbacks take over from the original driver. Candidate examples include the collapse of the West Antarctic and Greenland ice sheets, large-scale permafrost thaw, and the conversion of parts of the Amazon from rainforest to savanna. Their significance for policy is that they make gradual cost–benefit reasoning unsafe: past the threshold, stopping emissions no longer stops the change.