The downs and ups of global warming

If scientists knew about the greenhouse effect a hundred years ago, and if it as been obvious for decades that atmospheric C02 is on the increase, why didn't we hear alarm bells ringing sooner? To begin with, the temperature pattern of the twentieth century threw scientists off the trail. The global average rose dramatically from the 1890s through the 1930s, as noted by Callendar, then began to level off. The decades of warming brought a spate of public attention – “Is the World Getting Warmer?” asked a Saturday Evening Post headline of 1950 – but the global average dropped from the late 1940s into the mid-1970s. This made many scientists wonder whether the human-enhanced greenhouse effect – at that stage a phenomenon generally accepted by researchers but still little known by the public – might be counteracted by the cooling effects of soot and grit in the air or by some other unknown element. With the help of a few eloquent spokespeople, books and magazine articles played up the risk of a “snow blitz” that could bring about a new Ice Age, just as researchers were beginning to trace the impact of previous ice ages through ocean sediments. A few scientists who expressed concern in the 1970s over global cooling later came to believe that warming was the bigger concern – only to find themselves dodging their earlier statements, flung back at them by global-change sceptics. Yet even during the height of the global-cooling media craze, many scientists continued to quietly study the long-term risks posed by greenhouse gases.

As we now know, the cooling of the mid-twentieth century was only one step backward in a forward-stepping process. Temperatures began to climb again in the 1980s, and the greenhouse effect became a household phrase in 1988. NASA scientist James Hansen testified before the US Congress that he was “99-percent confident” that the summer's abnormal heat was part of a trend related to global warming. A single hot summer doesn't equal climate change, of course, but Hansen's comments – based on years of climate modeling and other evidence – resonated in the steamy air of Washington, DC.

Things accelerated in the 1990s, which scored the five warmest years in the century-plus record up to that point. The trend was only briefly interrupted when the eruption of the Philippines' Mt Pinatubo produced the largest volcanic cloud of the century in June 1991. Millions of tons of volcanic debris were thrown into the stratosphere, blocking sunlight and cooling the globe. A few months after Mt Pinatubo blew its top, Hansen predicted that a multi-year global cooldown would be followed by more records in the late 1990s. Nobody else made quite as flat a prediction, but many quietly shared Hansens belief, and it proved to be correct: the years 1997 and 1998 each set new global highs, and the worldwide trend played out in many local areas, especially at higher latitudes across the Northern Hemisphere. In the 1990s, central England saw about eight days per year whose average temperature was above 20°C/68°F. That's about double the rate observed in the preceding two hundred years. The first few years after 2000 have shown no sign of a planetary cooldown. The year 2005 was just as warm as the record-setting 1998 – and that was without any help from the gigantic 1997-98 El Nino, which gave global temperatures a temporary boost.

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Statistics are an abstract concept to most of us when we're freezing at the bus stop or over-heating in a room without air conditioning. How does global change translate into weather we can feel and see? The science of “downscaling” global wanning to your backyard is still a fairly crude enterprise. Even what seems to be the most obvious conclusion – you'll be warmer more often – can't be taken for granted. Just as some places didn't share in the warming of the twentieth century, it's possible that a few spots may find themselves cooler on average when the year 2100 rolls around.

Even if forecasters can't tell you where every individual raindrop will fell, they can let you know with confidence that there's a better chance of rain tomorrow than there was today. This is the sort of information – probabilities, possibilities, risks – that scientists generate in order to help policy makers figure out how we should respond to the global change threat Here are a few of the overall points to keep in mind. Some of them are widely accepted, while others are more controversial.

Global warming won't do away with cold snaps. In the US, Arctic outbreaks often produce a rash of editorial cartoons and people on the street asking, “Whatever happened to global warming?” Although models tend to agree that cold extremes will become less intense in a greenhouse-warmed climate, this still allows for plenty of up-and-down variations from day to day – in other words, weather.
Both floods and droughts may intensify. The reason for this apparent paradox is that, as temperatures go up, more water tends to evaporate from the ocean. That means, on average, more moisture is available in the air for making rain. Global precipitation is difficult to measure, since it's impossible to blanket the oceans with a network of fixed monitoring stations, and satellite measurements of rainfall are still being refined. However, the best estimates show an increase of as much as 5 to 10 percent in rain and snow amounts over Northern Hemisphere land areas in the past century. In the US, climatologist Tom Karl and colleagues have demonstrated that more of a given year's rainfall tends to fall in heavier bursts than it did a century ago Similar results have been compiled for many other parts of the world, with a few noteworthy exceptions (such as southwest Australia, which has been drying out over recent decades). More intense rainfall suggests that further flooding or more severe flooding is possible, although many other factors are involved, such as how cities are built and how watersheds are managed. As for droughts, what little water remains in the soil is more likely to evaporate when it's warmer – thus, the dry ground gets even drier. Some computer models hint that the most likely place for increased summer drought in the decades to come is the interior of continents, such as central Europe or the US Great Plains.
Overall, warming is likely to be more dramatic in the winter and at night This was the pattern of the last century; winters warmed more than summers did, and over land the nighttime lows warmed almost twice as much as did the daytime highs. In part, this is probably a side effect of extra moisture and clouds in the air.These help keep temperatures from dropping as much as they otherwise would, especially on cold winter nights. One result already in evidence has been a lengthening of the growing season in many areas. A European survey published in the journal Nature found that by the mid-1990s plants were blossoming an average of six days earlier than they were in the 1960s, while autumn colours tended to arrive about five days later. At some high latitudes, the growing season is now more than two weeks longer than in the 1950s.
Global dimming isn't likely to save the day. Since the mid-20th century, the amount of sunlight reaching land areas around the globe decreased by a substantial amount -10 percent or more in some areas.The most likely culprits are airborne particles, or aerosols, emitted directly from cars, factories and homes or created as pollutants mingle with sunlight. Because our experience of weather is dominated by daily and seasonal shifts among sunshine, cloudiness, and nighttime, it's doubtful that the world looks noticeably darker to the average person on the street The dimming hasn't been enough to knock out global warming, either, given that temperatures have continued to rise since the 1970s. Some studies show the dimming effect has weakened since 1990, though, as cleaner-burning technology helps reduce the output of sulphates and other short-lived pollutants responsible for dimming. It's much easier to keep aerosols out of the air than It is to cut back on carbon dioxide, so we can expect dimming to take a backseat to warming on the global scale.
Hurricanes appear to be gaining strength as global warming proceeds, but the specifics are still being debated. Today's global climate models don't have the resolution to track individual low-pressure centres in much detail, so they can't easily show whether such storms will change in number or strength as greenhouse gases increase. Despite 2005's bumper crop of Atlantic hurricanes, there is little evidence that hurricane counts have increased globally or that they will rise in a warmer climate. However, a larger fraction of hurricanes worldwide has been reaching intense levels since the 1970s, according to several studies in 2005-06 that triggered a lively research and policy debate. Computer models hint that warmer oceans may cause a continued increase in strength among the most powerful hurricanes. The jury is still out on the significance of such a trend for society, given the uncertainties of hurricane landfall locations – but even if hurricanes don't get worse, the growth in coastal settlement across hurricane-prone areas will only increase the risk of devastation and injury.
There's no particular reason to believe that tornadoes will increase or intensify. The upswing in tornado reports in the US and Europe across recent decades is believed to be mainly because more people are looking for twisters, and they're spotting more of the weak, short-lived ones that would have been missed in earlier times. Fortunately, the more violent tornadoes don't appear to be changing in frequency.