Changing nature in a warmer world

Posted: 19 February 2001

Author: Maya Pastakia

Media coverage of climate change has been characterised by both sensationalism and speculation. However, a number of recent scientific studies have emerged showing how various species around the world - from a variety of latitudes and environments - are altering their behaviour or distribution as a result of global warming. Maya Pastakia reports.

The clearest evidence for climate change comes from land temperature records kept since 1860. These show a fluctuating but steady rise in global temperature in most parts of the world. This rise in temperature also correlates closely with the increased levels of carbon dioxide in the atmosphere, which have shown a similar increase since the beginning of the industrial revolution in the mid-1800s.

Most scientists believe that changes in the global climate are being intensified by human activities, most notably the burning of fossil fuels which release greenhouse gases such as carbon dioxide and methane into the atmosphere. As the Intergovernmental Panel on Climate Change announced in carefully chosen words in 1995: "The balance of evidence suggests a discernible human influence on global climate."

Paleoclimatic evidence

One way of assessing how climate has changed over the years is to look at the paleoclimatic evidence. The Paleo (ancient) climate is climate that existed before humans began collecting instrumental measurements of weather with thermometers, rain gauges, barometers and anemometers, among others. Instead, paleoclimatologists use natural environmental (or "proxy") records to infer past climate conditions, such as tree-rings, skeletons of tropical coral reefs, frozen in glaciers and ice caps, and the sediments of lakes and oceans. By analysing these sources, scientists can extend our understanding of climate far beyond the 140-year instrumental record, providing a longer time frame of hundreds to tens of thousands of years.

Ice core evidence

One source for a paleoclimatologist is ice buried deep beneath the earth's surface. In 1999, a joint US-French-Russian team completed drilling at Vostok station in East Antarctica, one of the coldest places on Earth.Ice core© BBCBy analysing the composition of ice cores from many kilometres deep into the ice sheet, they have been able to build up a record of past temperatures and levels of carbon dioxide and methane - two of the most important greenhouse gases - in the atmosphere going back 420,000 years. This period covers the last four glacial-interglacial cycles, where the Earth has cooled and then warmed over many thousands of years. First, the scientists discovered that there is more CO2 and methane in the atmosphere than at any point during the past 420,000 years. In 1999, for example, levels of CO2 were between 360 and 365 parts per million. The scientists had to look back 300,000 years - when the CO2 level was 300 parts per million - to find a concentration which even came close to this. This shows that despite volcanic eruptions and other natural phenomena which release greenhouse gases into the atmosphere, humans appear to have pushed CO2 and methane in the atmosphere to unprecedented levels.

The team then investigated the link between the amount of CO2 and methane in the atmosphere with temperature fluctuations. They found that during warm, inter-glacial phases such as the one we are currently in, greenhouse gases and temperature are very closely - and positively - correlated. Basically, as CO2 increases in the atmosphere, the world warms. This is an important discovery because it indicates that the greenhouse effect, and not some other occurrence such as sunspots or changes in the Earth's orbit around the sun, is largely responsible for the current warming of the climate.

Sinking feeling

Today, many other scientific investigations have shown a distinct link between changes in the environment and our present warmer world.

Point Barrow, the most northerly and bleakest spot in the Arctic continent is not a place you would normally associate with global warming. But this bitterly cold part of the globe is warming up. Temperatures in the Arctic have been rising by as much as 1.5 degrees centigrade per decade, while the ice cover is now reducing by between three and five per cent per decade. Sea ice decline is the most significant impact of climate change on the Arctic ecosystem. One immediate result is the detrimental impact on the polar bears in the region.

For 30 years, Dr Ian Stirling of the Canadian Wildlife Service has been researching polar bears in the Western Hudson Bay, near the town of Churchill. For the last 20, he has compiled detailed records of their health, weight and birth rates. These historical records are vital if scientists wish to understand the changing influence of the environment on the bears.

According to Dr Stirling, due to increasing temperatures over recent years, the sea ice in Hudson Bay is breaking up earlier - about a week to two weeks earlier - than it used to 20 years ago. For the polar bears this severely affects their feeding patterns. In the weeks before the ice breaks up, the bears hunt seal cubs on the sea ice in the spring and must consume as many of these young seals as they can. The best hunting falls from April onwards when ringed seal pups are born. If the ice breaks up early, then the bears have a shorter period of time to fatten up before their four-month period of fasting in the summer months when the ice has melted and they wait on land for it to refreeze.

Polar bears are under threat due toearly melt of sea ice in the Arctic© Thomas D. Mangelsen/Still Pictures Comparing the timing of the ice break-up in Hudson Bay with the condition of the polar bears, Dr Stirling has found that in warmer years, when the ice breaks up earlier, the bears come ashore lighter and in poorer condition, with little fat to live off for the four months that they are on shore. Fewer cubs are born and the survival rate of cubs declines.

Dr Ian Stirling warns that if the present climatic trend in the Arctic continues for another 10 or 20 years, "it's inevitable that the bears population will decline."

Migrant butterflies

Many types of wildlife depend on natural signals, such as temperature and day length, to time their life cycles. Because of climate change, those species which depend on temperature may migrate or breed earlier. A number of studies in the temperate zone provide such evidence. For instance, in a sample of 35 non-migratory European butterflies, twenty-two have shifted their range northward by 35-240 km during the last 50 years, while one has shifted to the south.¹

In Britain, seasonal and distributional changes have been recorded by Ken Durrant and his colleagues of the Norfolk and Norwich Naturalists Society. They came across the first speckled wood butterfly Parage aegeria, which is extending its range northwards, on a nature walk on Beeston Common in 1990. Since then, the butterfly has spread all over Norfolk. Other migrant species of butterfly more at home in southern Europe are now starting to thrive in Britain, including The Painted Lady, The Comma, Gatekeeper and Small Skipper.

ButterflyOrange tip butterfly is emerging 20 days earlier in BritainAccording Dr Tim Sparks of the Institute for Terrestrial Ecology who specialises in phenology (the study of the timing of natural events relating to arrival, reproduction and emergence), "increased spring temperatures have resulted in markedly earlier appearance and an increased number of generations." He assets that in Britain there was "little doubt" that milder winters and warmer summers were helping caterpillars develop and changing the behaviour of adult butterflies.

As well as migration changes for butterflies, birds are also changing the timing of their egg-laying. A study by Humphrey Crick of the British Trust for Ornithology analysed 92,828 bird-nesting records, compiled from 1939 to 1995, for 65 breeding-bird species across the UK. Over 25 years, from 1971 to 1995, 63 per cent of the species showed a tendency to nest earlier. For nine of the species - such as wrens, great and blue tits, willow warblers, green finch and blackcap - the tendency was statistically significant. Only one species showed a significant tendency to lay eggs later in the year.

Many species of plant in the UK are also being affected by climate change. Dr Tim Sparks research into tree-leafing times, sometimes using records kept by amateur naturalists as far back as the mid-eighteenth century, shows how the leafing dates of more than 12 species of tree in the UK are closely correlated with temperature. Indeed, a one degree centigrade increase results in earlier leafing by 5-7 days.

Changing course

One hundred miles west of Detroit, the wetlands resound to the call of Sandhill Cranes. The response of these birds to climate change is of special interest because of the immense time that they've lived on earth. They evolved in the Age of the dinosaurs, 60-70 million years ago.

According to Dr Terry Root of the University of Michigan who has migration records of the birds going back 30 years, "the Sandhill Cranes fly down to Florida, and that's where they over-winter because it's warm enough down there. And then in the summer they take pretty much the same path back. Because the fall is warmer now, there is a tendency for them to be leaving about 20 days later in the fall than they used to."

Having established that among birds over-wintering in the South, some will fly north much earlier than they have in the past, the next question is what triggers their departure. Dr Terry Root is convinced that global warming is the most likely explanation for their early arrival: "What in the world can be triggering things down in Central America, Middle America, over in Europe, down in Africa, to be moving everything earlier? It has to be global and to me the thing that really links up is global climate change. The circumstantial data is just piling up and piling up and piling up. It doesn't prove it, but it strongly suggests it."

Changing altitudes

One dramatic example of the impact of climate change is to be found in the Monteverde Cloud Forests of Costa Rica. Alan Pounds, a conservation biologist associated with the University of Miami, Florida, has lived and studied these forests for the past 20 years. He has been especially interested in the disappearance of the famed Golden Toads which are only found in the cloud forests of Monteverde. Once abundant, only one male has been seen since 1989, since when the species had mysteriously vanished.

One explanation for the fate of the Golden Toad and the other amphibians, has been the suggestion that they have been attacked by fungal infections or parasites. But why should so many species fall victim at the same time? Most of them are assumed to be at least locally extinct and, in the case of the golden toad, that means globally extinct. What puzzled Pounds in his search for an explanation was the sure knowledge that his protected study area remained pristine. Despite this, frogs and another creatures were disappearing.

Golden toad spawning© And P. FogdenFor Pounds, whatever was doing the damage had to be something basic, and since nothing is more important to Amphibians than water, he began looking at local weather patterns. After the 1987 population declines, he was fortunate to come across weather data collected by another Monteverde resident, John Campbell, which showed that the major amphibian and reptile die-offs occurred during periods when dry days were common.

With the analysis of Campbell's daily weather records taken since the mid-1970s of the same area of Monteverde as Pounds study area, Alan Pounds was able to demonstrate that the number of dry days in the winter dry season increased steadily between 1973 and 1998. He also showed that the number of dry spells lasting five days or more had markedly increased. For frogs and toads that could well have been the fatal difference.

Then he discovered the work of Michael Falcon, an Oxford-trained ornithologist. His records show how birds within Pounds' study area have changed as 15 species typical of lower altitudes - such as the Blue Crowned Mots Mots, Masked Tityras and Keel-billed Toucans - have shifted upslope and began breeding in the cloud forest, side-by-side with resplendent Quetzals. According to Pounds, "We see a lot of birds that normally don't inhabit cloud forest that are moving up the mountain slope."

"We have to look deeper for a common denominator, and the patterns that we are looking at strongly suggest that climate is that common denominator."

Within the Monteverde Cloud Forest, every altitude has different conditions; drier and warmer in the foothills, cooler and wetter towards the summit where the true cloud forest is found. And each climatic layer has its specific and highly adapted range of plants and animals. According to climatologist, Stephen Schneider, when the earth warms up by a few degrees it lifts the cloud deck up by several hundred metres, encouraging some species to move up with it.

Alpine shift

This upward shifting pattern is also apparent in The Alps. Changes to alpine flora over the past 70 to 90 years show that even moderate warming causes plant communities to move up-slope. As temperature has increased, alpine flora has migrated to higher elevations where it is more cool. This movement, however, is occurring at a much slower rate than changes in temperature, and new areas are being colonised more slowly than the rate at which the plants' original areas are becoming unsuitable. The area of suitable habitat for these species is therefore diminishing in size.

Coral bleaching

Some impacts of climate change on the natural environment can be sudden and catastrophic. By now it is clear the planet's coral reefs are under serious threat from warming seas. A temperature rise of just one or two degrees can wipe out corals which have existed for thousands of years.

Coral reefs are the most diverse of all ecosystems, richer even than tropical rainforests. Although they occupy less than one per cent of the vast space taken up by the oceans, they support a quarter of all marine fish species.

In 1998, the most dramatic warming of the world's oceans occurred during the severe El Niño event. Large areas of the Pacific and Indian Oceans were affected by higher than normal water temperatures and increased solar radiation. This short-term warming during the summer of 1998 was overlaid on a long-term warming of the oceans due to climate change. The result was catastrophic coral bleaching the world over.

bleached coralBleached coral© Norbert Wu/Still PicturesBleaching occurs when coral polyps become stressed and expel the colourful, symbiotic algae which live within their skeletons, leaving entire reefs white, or 'bleached'. Deprived of the food the photosynthetic algae provide their hosts, the corals soon die. Wave and storm damage eventually turn once intricate and beautiful structures into rubble. Bleaching can be caused by a variety of external stresses such as sediment in the water or change in water acidity. The most common cause, however, is an increase in sea temperatures.

In The Maldives in the middle of the Indian Ocean, one of the most pristine and unpolluted coral environments in the world, 80 to 90 per cent of its reefs have been affected as a result of this warming. Dr Tom Goreau of the Global Coral Reef Alliance, offers the following explanation: "In 1998 we saw corals a thousands years old that have survived hundreds of El Niños in the past, die, so we know that there's never been an El Niño like this in a thousand years, otherwise these corals would still be around. What we think is happening is that the El Niño is introducing these temperature oscillations in some places, but the baseline of global warming on which those oscillations are superimposed is resulting in each El Niño being just a little bit higher than the one before. And so, even in those places where the El Niño is a proximate cause of high temperature, we have to say that global warming is the ultimate cause."

After bleaching, the coral begins to die. If the water temperature quickly returns to normal, the corals will recover. But if the temperature remains high for more than a few weeks, the bleached coral dies. For The Maldives, the reefs generate a huge source of food for local people and income from tourism. Loss of its protective reef barriers and rising sea levels could spell the end of this island nation.

Sea change

As sea surface temperatures around the world rise, the distribution of marine species is changing. Chuck Baxter of the Hopkins Marine Station has been studying the intertidal zone (the area between the upper limit of the high tide and the lowest limit of the low tide) in California since 1974. He began noticing that species communities in the zone were beginning look more and more like the communities that he had been familiar with 200 miles south in southern California.

He also discovered from the records that sea temperatures during the warmest part of the year were over two degrees warmer than in 1930.

"There's about a dozen more southern species which either didn't exist or were extremely rare at Hopkins which have become abundant in the transect area," says Baxter. Also noted in the study was the decline of a number of northern species, which used to be dominant.

Antarctic warning

It seems that no region in the world has been unaffected by temperature change. In the case of the Antarctic peninsula, which extends far up into the South Atlantic, warming temperatures have resulted in early thaws and, as in the Arctic, marked decreases in sea ice.This early thaw has affected the colony of Adelie penguins which live in the sea ice zone which surrounds Antarctica. They are found only where sea ice lasts well into the spring thaw.

Scientist, Dr David Ainley, who studied the penguins, describes their relationship with sea ice "like a duck's with water." In winter, the penguins dive to catch krill in cracks in the ice overlying places where nutrient rich water comes to the surface. When sea ice is reduced, survival of adult and young penguins is poor, as suitable feeding sites are too scarce or poor.

The early thaw is resulting in the disappearance of Adelie penguin colonies which are being replaced by sea ice intolerant penguin species such as the Gentoos and Chinstraps.

Fact or fiction?

So what if a butterfly does turn up somewhere unexpectedly, or the Sandhill Cranes migrate later? These trifling facts are now more grandly known to science as phonological data and they, along with many other similar scientific studies, are conclusive in demonstrating what global warming is doing to nature's calendar. In a letter to a fellow naturalist, Charles Darwin commended him on his meticulous attention to what people are pleased to call trifling facts: "These are the very facts," wrote Darwin, "that make me understand the workings or the economy of nature." To say that the dilemma posed by polar bears and a seas of dying corals is the result of 'climate change' is a trivialisation of what is happening on earth today. What we are witnessing is a profound change in the nature of planet earth.

Maya Pastakia is Assistant Editor of People & the Planet.This article is based on the BBC documentary, Warnings from the Wild.Related link:World View of Global Warming - The photographic documentation of climate change