NEW YORK: In Sweden and Norway, the treeline is marching northward and uphill as the snowline recedes. In the Arctic, the polar bear finds its habitat shrinking. Elsewhere in the northern hemisphere, animals are slowly moving north to escape rising temperatures. Behind the silent movement hides a disturbing story that we had better take note of before it is too late. If the present warming trend continues, rising seawater will claim coastal cities all over the world.
Animals have no choice but to move, since their survival is at stake. Recently after appearing on television to discuss climate change, I received an e-mail from a man in northeast Arkansas about his observations of the armadillo: “I had not seen one of these animals my entire life, until the last ten years. I drive the same 40-mile trip on the same road every day and have slowly watched these critters advance further north every year and they are not stopping. Every year they move several miles."
The mobility of armadillos suggests that they have a good chance to keep up with the movement of their climate zone, to be one of the surviving species.
Other species have greater problems. Of course, climate fluctuated in the past, yet species adapted and flourished. But now the rate of climate change driven by human activity is reaching a level that dwarfs natural rates of change. If climate change is too great, natural barriers, such as coastlines, spell doom for some species.
Studies of more than 1,000 species of plants, animals, and insects, found an average migration rate toward the North and South Poles of about four miles per decade in the second half of the 20th century. That is not fast enough. During the past 30 years the lines marking the regions in which a given average temperature prevails, or isotherms, have moved poleward at a rate of about 35 miles per decade.
As long as the total movement of isotherms toward the poles is much smaller than the size of the habitat, or the ranges in which the animals live, the effect on species is limited. But now the movement is inexorably toward the poles, totaling more than 100 miles in recent decades. If emissions of greenhouse gases continue to increase at the current rate – "business as usual" – then the rate of isotherm movement will double during this century to at least 70 miles per decade. If we continue on this path, a large fraction of the species on Earth, as many as 50 percent or more, may become extinct.
The species most at risk are those in polar climates and the biologically diverse slopes of alpine regions, literally pushed off the planet. A few species, such as polar bears, no doubt will be "rescued" by human beings, but survival in zoos or reserves will be small consolation to bears or nature lovers.
In the Earth's history, during periods when average global temperatures increased by as much as 10 degrees Fahrenheit, there have been several "mass extinctions," when between 50 and 90 percent of the species on Earth disappeared forever. In each case, life survived and new species developed over hundreds of thousands of years – but the life that survived was dramatically different from that which dominated before. The most recent of these mass extinctions defines the boundary, 55 million years ago, between the Paleocene and Eocene epochs. The evolutionary turmoil associated with that climate change gave rise to a host of modern mammals, from rodents to primates, which appear in fossil records for the first time in the early Eocene.
If human beings follow a business-as-usual course, continuing to exploit fossil fuel resources without reducing carbon emissions or capturing and sequestering them before they warm the atmosphere, the eventual effects on climate and life may be comparable to those at the time of mass extinctions. Life will survive, but on a transformed planet. For foreseeable human generations, the world will be far more desolate than the one in which civilization flourished during the past several thousand years.
The greatest threat of climate change for human beings lies in the potential destabilization of the massive ice sheets in Greenland and Antarctica, a catastrophe that would be as irreversible as the extinction of species.
Future rise in the sea level depends, dramatically, on the increase in greenhouse gases, which will largely determine the amount of global warming.
To arrive at an effective policy we can project two scenarios concerning climate change. In the business-as-usual scenario, annual emissions of CO2 continue to increase at the current rate for at least 50 years. In the alternative scenario, CO2 emissions level off this decade, slowly decline for a few decades, and by mid-century decrease rapidly, aided by new technologies.
The business-as-usual scenario yields an increase of about 5 degrees Fahrenheit of global warming during this century, while the alternative scenario yields an increase of less than 2 degrees Fahrenheit during the same period.
The last time that the Earth was five degrees warmer was 3 million years ago, when the sea level was about 80 feet higher.
In that case, the world would lose Shanghai, Tokyo, Amsterdam, Venice and New York. . In the US, 50 million people live below that sea level. China would have 250 million displaced persons. Bangladesh would produce 120 million refugees, practically the entire nation. India would lose the land of 150 million people.
A rise in sea level, necessarily, begins slowly. Massive ice sheets soften before rapid disintegration and melting occurs and sea level rises. The Earth's history reveals cases in which sea level, once ice sheets began to collapse, rose 1 meter every 20 years for centuries, calamity for hundreds of cities throughout the world.
Satellite images and other data have revealed the initial response of ice sheets to global warming. The area on Greenland in which summer melting of ice took place increased more than 50 percent during the last 25 years. The volume of icebergs from Greenland has doubled in the last 10 years.
The effect of this loss of ice on the global sea level is small so far, but accelerating. The likelihood of the sudden collapse of ice sheets increases as global warming continues. For example, wet ice is darker; thus, as ice sheets continue to melt they absorb more sunlight and melt even faster.
The business-as-usual scenario, with 5 degrees Fahrenheit global warming and 10 degrees Fahrenheit at the ice sheets, would certainly lead to their disintegration. The only question is when the collapse will begin. The business-as-usual scenario, which could lead to an eventual sea level rise of 80 feet, with 20 feet or more per century, could produce global chaos, leaving fewer resources with which to mitigate the change in climate. The alternative scenario, with global warming under 2 degrees Fahrenheit, still produces a rise in the sea level, but the slower rate allows time to develop strategies for adapting to the changes.
The Earth's creatures, save for one species, do not have thermostats in their living rooms that they can adjust for an optimum environment. But people – those with thermostats – must take notice, and turn down the world’s thermostat before it is too late.