The Latest Complication in Our Changing Weather

As climatologists learn how global warming is changing the world’s weather, they are also having to consider a variable that is complicating an already complicated process.

Global dimming is the effect on weather caused by particles of atmospheric pollution called aerosols. As human activity increases the concentrations of carbon dioxide and methane — the principal causes of global warming — we are also releasing into the air huge quantities of sulphur and nitrogen compounds, along with soot and other particulates. While global warming is causing the planet to heat by reducing the amount of radiant energy that escapes back into space, these aerosols cause a cooling by reflecting sunlight before it reaches the Earth’s surface. In other words, global dimming is counteracting the effects of global warming, making an incredibly complex subject even more complex and unpredictable.

Scientists have been noting this dimming process for decades. Israel has measured a 22% decline in the intensity of sunlight since 1950. Similar declines have been measured elsewhere: Australia 9%, USA 10%, Britain 16%, Russia 30%.

A four-year study in the Maldives dramatically illustrates the effects of this dimming. The long string of islands stretching north-south across the equator just below India is unique because the northern half receives pollution-laden trade winds blowing southward from India, while the southern half receives relatively clean winds blowing northward from Antarctica. The 3 km-thick pall of polluted air from India has reduced the amount of sunlight striking the northern islands by 10%. The effect is caused partly by direct reflection, and partly by increased cloud formation — the presence of large amounts of aerosols, on which water vapour condenses, aids cloud formation.

But a unique opportunity to study global dimming arose during the three days of September 11th to 13th, 2001, when the US grounded all internal flights because of terrorist attacks. Scientists have wondered about the cooling effects of the large number of condensation trails from high flying commercial aircraft that cris-cross the world’s continents and oceans. During those critical three days in America, the 5,000 weather stations across the country measured an average temperature increase of 1°C — a graph indicates a dramatic spike for only those days.

The sobering implication is that we live in a world of global warming in which some of the heating effects are being masked by global dimming. The information arising from just the 9/11 incident seems to be confirmed by broader studies showing that, as we clean up air pollution, temperatures unexpectedly rise in tandem.

Changing sunlight and temperature can have significant effects on weather. According to one theory, a band of increased pollution near the equator reduced the regional heating that was necessary to draw water-laden tropical clouds northward to a swath of Africa dependent on annual rains. The result was a 20-year drought in the Sahal region during the ‘70s and ‘80s, an event that caused the death of about one million people and the plight of some 50 million others.

Throughout many parts of Asia, the dimming process has the opposite effect. Instead of droughts, aerosols combine with rising global temperatures and atmospheric water vapour levels to increase the intensity of monsoons. About 3.5 billion people are affected by a trend that is bringing greater rainfall for shorter periods of time. The result is stressed food production, and increased floods and landslides.

Which brings us to British Columbia’s recent weather. While no one can know the precise causes of any individual weather event, climate models predict that global warming will bring wetter winters and drier summers to coastal BC— higher ocean and air temperatures will generate more extreme storms with greater moisture content. But our extreme rainfall events of the last several months might also be aided by aerosols.

Dr. Renyi Zhang and a team of atmospheric scientists from Texas A&M
University are studying the large amounts of high atmospheric pollution
that are blowing eastward across the Pacific from China to BC (Globe &
Mail, Mar. 6/07). They suspect that these aerosols are generating
clouds, changing weather patterns and increasing the intensity of
rainfall. The scientists haven’t noticed a similar change occurring over
the Atlantic, presumably because Europe is reducing its atmospheric
particulate pollution rather than increasing it.

If the science of global dimming is correct, we have added another
massive variable to the uncertainties we’ve created with climate change
We now find ourselves in the dilemma where greenhouse gases and aerosols
are having opposite effects on the warming trend we are trying to
prevent. But, while the cooling counteracts the warming, we want neither
the pollution nor the extreme weather. So what do we do to get ourselves
out of this deepening mess?

Perhaps our dilemma is summarized best by James Lovelock. The senior British environmentalist and originator of the acclaimed Gaia Theory has also become an elder philosopher. In his recent book The Revenge of Gaia, he thinks we may have created for ourselves “the ultimate form of slavery” (p. 152). “The more we meddle with the Earth’s composition and try to fix its climate,” he writes, “the more we take on the responsibility for keeping the Earth a fit place for life, until eventually our whole lives may be spent in drudgery doing the tasks that previously Gaia had freely done for over three billion years. This would be the worst of fates for us...”.

So the next time the heavy rains come and the strong winds blow, we have something more to think about than the weather.