As temperatures soar, there is now little doubt that our climate is changing, but is human activity to blame? Brendan McWilliamslooks at the evidence, and writers report on unusual weather patterns across Europe this winter.
The spring, the summer,
The chiding autumn, angry winter, change
Their wonted liveries; and the mazed world,
By their increase, now knows not which is which.
- Shakespeare: A Midsummer Night's Dream.
Clearly, Shakespeare was familiar with the signs of climate change, although the changes in his time were not on the same scale, perhaps, as those to which Nature may need to adapt in the lifetime of the present generation.
The seasons havealtered. In Europe, for example, studies show that spring events such as leaf unfolding have advanced by about six days during the last 40 years, whereas autumn events such as leaf colouring now occur, on average, about five or six days later; the combined effect is that the average growing season in Europe has lengthened by around 11 days since the early 1960s.
A similar picture emerges on the other side of the Atlantic. These changes can be attributed almost entirely to changes in the average temperature, particularly winter temperature, since other variables such as soil composition, water supply and biological factors have, by and large, remained unchanged.
These phenological results suggest a changing world. Corresponding observations of the expansion and contraction of the habitual ranges of various animal species, changes in their physical or reproductive behaviour, and the timing of, for example, frog spawning or the average first appearances of butterflies or migratory birds, are also symptomatic of trends in local average temperature throughout the cycle of the year, and hence of incipient climatic change.
But the climatological facts themselves are now well known. During the 20th century, the average temperature of this planet increased by about 0.6 degrees, the largest change in any 100-year period for at least 1,000 years. This warming has accelerated quite dramatically in the last two decades, a trend which, in turn, has produced more interesting statistics.
The 10 warmest years in the last millennium, for example, have all occurred since 1990, the two contenders for the hottest year on record being 1998 and 2005. These two, the experts tell us, are "statistically indistinguishable" from one another, but there is an important difference between the two. 1998 was the year of one of the strongest El Niño events of recent decades; this periodic warming of the tropical waters of the Pacific Ocean every three to seven years always gives global temperatures a little boost, but the more recent year, 2005, achieved record status with no help whatever from El Niño.
LOCALLY, TOO, WEsee signs of unprecedented warmth. In Britain, the period from May to September last year was the warmest such period for at least 300 years; here in Ireland, 2006 was the warmest summer generally since those halcyon days of 1995, and in parts the warmest for well over a century; and, as we know, autumn and winter so far have been exceptionally mild. A similar picture emerges across the Atlantic, with the year 2006, albeit only sixth in line worldwide, being the hottest year since records began in the United States.
Global warming, then, if only by definition, must be accepted as reality. But can we attribute these highly unusual conditions directly to the enhanced greenhouse effect and, beyond that, to human causes? In the early days of the current trend for rising global temperature - during, say, the 1980s and early 1990s - many climate scientists were sceptical of this conclusion. They felt that the anomalies observed might well be due the natural variability of the climate of our planet. Natural climatic change happens for a variety of reasons, such as changes in the thermal output of the Sun, variations in the eccentricity of the Earth's orbit and the tilt of its axis of rotation, and volcanic activity emanating from the Earth itself.
But, year by year, the clear, continuing trend has become more and more persuasive. A pattern has developed where global temperatures are consistently above the levels of a quarter of a century ago; where temperature records are broken regularly; and where the months and years are warmer than their predecessors with increasing frequency. No one now denies that planet Earth is physically getting warmer; more significantly, it has become almost Luddite in scientific circles to deny that human activity is the cause.
The most widely respected authority on these matters is the Intergovernmental Panel on Climate Change (IPCC), a United Nations body comprising several hundred of the world's top climate scientists whose collective objectivity is universally accepted. The IPCC was established in the late 1980s, and it is interesting to trace how its conclusions have evolved.
In 1990, the IPCC's first report was cautious, stressing that the observed increase in global temperature was "within the bounds of natural climatic variability". After considering the evidence during the following five years, however, there was a decisive shift in emphasis; in 1995, the IPCC concluded that "the balance of evidence suggests a discernible human influence on global climate".
BUT BY 2001the panel's tone was almost strident; it concluded that the observed increase in global temperature was "unprecedented in the last 10,000 years", and could only be plausibly explained by taking account of human influence. It further concluded that other factors affecting the global climate - changes in radiant solar energy, or volcanic dust thrown into the atmosphere from time to time - should have made the world cooler, not warmer, during the previous two decades. Its verdict, in plain language, was that humanity was guilty beyond all reasonable doubt.
In addition to giving us a perspective on the past, IPCC has a leading role in arriving at a consensus on the future, and here its conclusions are equally alarming. The experts analyse a range of possible scenarios, depending on how the world may respond in terms of controlling greenhouse gas emissions, and conclude that the average global temperature is likely to increase by anything from 1.5 degrees to nearly 6 degrees in the present century. There is general agreement that the social, political and ecological consequences of a further rise of 2 degrees in global temperature would be serious indeed; but the consequences of the higher figure are recognised as nothing short of catastrophic for the planet.
The IPCC will produce a new report this year and, needless to say, its findings are eagerly, even fearfully, awaited by the decision-makers of the world.
HERE IN ANIreland rocked by devastating gales for what seems like months on end, it might be tempting to ascribe our current woes to global warming. But this, at the very least, is premature. It is true that an increase in storminess in the middle latitudes is not inconsistent with a greenhouse world, but long spells of very windy weather have always been a feature of our winter climate; they have occurred before, and will occur again, without any help from extra carbon dioxide in the atmosphere.
More to the point is the fact that the UK Met Office has recently published its thoughts on how this year, 2007, may fit into the pattern which has evolved in recent decades. Pointing out that El Niño conditions have developed over the Pacific in recent months and are likely to persist for half the current year, it concludes that there is a high likelihood of 2007 surpassing 1998 to become the Earth's hottest year on record.