Planethopia

In June 1783, lava and gases began pouring from the Laki fissure in Iceland in one of the biggest and most devastating eruptions in history. Poisonous gases and starvation killed a quarter of Iceland’s population. The effects of the eight-month-long eruption were felt further afield, too. In the rest of Europe, a scorching summer of strange fogs was followed by a series of devastating winters.

At the time, French naturalist suggested the eruption might be to blame, but two centuries passed before scientists started to work out how gas and dust from volcanoes affect climate. The main culprit is sulphur dioxide, which has a cooling effect. Laki pumped an estimated 120 million tonnes of the stuff into the atmosphere, cooling the northern hemisphere by as much as 0.3 C over the next few years.

Nowadays, we are pumping out amounts of sulphur dioxide each year comparable to Laki’s emissions. Human emissions rose rapidly over the 20th century, peaking at an estimated 70 million tonnes a year in the 1990s as developed countries cleaned up their act. Even such huge amounts, however, have not been enough to stop global warming: the cooling effect has been more than offset by the warming effect of carbon dioxide and other pollutants.

We are only now beginning to understand the effects of some of those other pollutants. One of the major players is black carbon, produced by the burning of everything from dung to diesel. Some recent studies suggest it is one of the biggest causes of warming after CO2 in the short term, contributing to the rapid warming in the Arctic and the melting of Himalayan glaciers.

These findings mean we face both a danger and an opportunity. When China and India reduce their sulphur dioxide emissions, the rate at which the planet is warming will rise dramatically. Satellite measurements show that China is already making headway. As a result, the rate of warming could increase from the current 0.2 C per decade to 0.3 or 0.4C per decade. Such rapid change would make it much harder for adaption of people and wildlife.

On the plus side, we could head off this dramatic speed-up in warming over the next few years by tackling black carbon and some of the other short-lived pollutants that are helping to heat up the planet. This would buy us more time to reduce our dependence on fossil fuels.

Since the industrial age got under way, we have been pumping ever more pollutants into the atmosphere; not just gases like CO2, but also substances that form fine particles, or aerosols. The result is often visible in the form of a brown haze covering cities or even entire countries. The quantity of pollution is so vast that the amount of sunshine reaching Earth’s surface has declined by as much as 10% in places, a phenomenon known as global dimming.

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