April 19, 2012 - Throughout much of the Pleistocene era, which began 2.5 million years ago, many of the world's large mammals survived periods of glaciation and deglaciation by moving across a landscape devoid of humans. Then as the Pleistocene drew to a close at the end of the last Ice Age — some 20,000 to 12,000 years ago — creatures such as the wooly mammoth had to confront not only shrinking habitat caused by climate change. They also faced thousands of humans with stone-tipped weapons, a one-two punch that led to the extinction of dozens of so-called megafauna species, including the wooly mammoth, across Eurasia and North and South America.
Now, with 7 billion people on the planet — heading to 10 billion — and with greenhouse gas emissions threatening more rapid temperature rises than the warming that brought the last Ice Age to an end, the many millions of living things on Earth face an unprecedented squeeze. Is a wave of extinctions possible, and if so, what can we do about it?
The late climate scientist and biologist Stephen Schneider once described this confluence of events — species struggling to adapt to rapid warming in a world heavily modified by human action — as a "no-brainer for an extinction spasm." My colleagues Barry Brook and Anthony Barnosky recently put it this way, "We are witnessing a similar collision of human impacts and climatic changes that caused so many large animal extinctions toward the end of the Pleistocene. But today, given the greater magnitude of both climate change and other human pressures, the show promises to be a wide-screen technicolor version of the (by comparison) black-and-white letterbox drama that played out the first time around."
The magnitude of the threat was first quantified in a 2004 Nature study, "Extinction Risk from Climate Change." This paper suggested that in six diverse regions, 15 to 37 percent of species could be at risk of extinction. If those six regions were typical of the global risk, the study's authors later calculated, more than a million terrestrial and marine species could face extinction due to human encroachment and climate change — assuming conservatively that 10 million species exist in the world. Headlines around the world trumpeted the 1 million figure.
Whether that scenario will unfold is unclear. But signs of what is to come are already all around us: nearly 100 amphibian species in South America vanishing in a disease outbreak linked to climate change, large areas of western North American facing massive die-offs of trees because of warming-driven beetle outbreaks, and increasing loss of coral reefs worldwide because of human activities and coral bleaching events driven by rising ocean temperatures. Most of the world's biologically unique areas have already lost more than 70 percent of their high-quality habitat.
The world community has the power to greatly reduce the prospect of an extinction spasm by lowering greenhouse gas emissions and launching large-scale conservation and forest preservation programs that both slow global warming and provide a sanctuary for countless species. But progress on these fronts is slow, and pressure on the world's biodiversity remains relentless.
An important part of the solution is preserving the ability of species to move across a changing landscape. Before humans, species responded to climate change by migrating, sometimes long distances, to track their preferred climatic conditions. Fully natural landscapes were conducive to these movements, with even slow-dispersing plants shifting the heart of their range on continental scales. The mechanisms of these changes are still being worked out, but we know they happened: Insects once found in Britain are now found only in the Himalayas, and centers of oak distribution have moved from the Mediterranean to Central Europe and from Georgia to Pennsylvania.
Recent studies have shown that migration was an important method for species to cope with rapid climate change as far back as 55 million years ago, a period known as the Paleocene-Eocene Thermal Maximum, or PETM. Then, for reasons that are still not entirely clear, vast amounts of greenhouse gases were released into the atmosphere and oceans, leading to an increase in global temperatures of 4 to 9 degrees C (7 to 14 degrees F) in less than 10,000 years. Geological and fossil studies, using techniques such as stable isotope analysis, show major extinctions, the evolution of new animals and plants, and the migration of species on a large scale.
Now, however, landscapes are crowded with human uses. Cities, urban sprawl, and agriculture take up huge areas. Freeways and roads create long linear obstructions to natural movement and present a patchwork of obstacles that are a severe challenge to species' natural modes of shifting to track climate. To unravel these future responses requires understanding of past response, modeling of future response, and insights from changes already underway.
To date, marine systems have experienced the most extensive impacts of climate change. From coral bleaching to melting sea ice, marine systems are changing on global and regional scales. Coral bleaching occurs when water temperatures exceed regional norms, causing corals to expel symbiotic micro-organisms from their tissues, ultimately leading to morbidity or death. Bleaching has exterminated some coral species from entire ocean basins. Global extinctions may follow as temperatures continue to rise.
Corals face a second threat from acidification as CO2 builds up in the atmosphere and oceans, which prevents corals and many other marine organisms, including clams and oysters, from forming their calcium carbonate shells. Overall, the evidence suggests that the world's roughly 5 million marine species face as severe threats from climate change as their terrestrial counterparts.
On land, tropical biodiversity hotspots in places such as the Amazon and the rainforests of Indonesia and Malaysia are especially at risk. All global climate models now show significant future warming in the tropics, even if more muted than warming at high latitudes. Tropical animals, insects, and plants are tightly packed along climatic gradients from lowlands to mountaintops, and these organisms are sensitive to changes in temperature and rainfall. Already, scores of amphibians in South America have disappeared as a warmer, drier climate has led to outbreaks of disease such as the chytrid fungus. At the same time, large areas of tropical forest are being cleared for timber, ranching, and farming such crops as soybeans and oil palm.
While these circumstances point to likely biological extinctions in the oceans and on land, functional extinctions may be of even greater concern. Functional extinctions occur when a species' population crashes to the point at which its functional roles within an ecosystem collapse. Functional extinction always accompanies biological extinction, but can happen before biological extinction is complete. Corals, for example, may be lost from huge areas, resulting in ecosystem conversion from coral reef to algal mat while some coral individuals still persist in isolation. Bark beetle outbreaks driven by climate change have killed tens of millions of trees from Colorado to Canada, causing functional extinctions of lodgepole pine across large areas of western North America. The repercussions of these tree losses are felt in a host of ways, from declining food for keystone species, such as bears, to increased risk of fire.
The good news is that we know what to do to reduce the risk of extinction due to climate change and other human activity. The first and most obvious step is to do something about global warming itself by reducing greenhouse gas emissions. The second part of the solution is to create smarter conservation strategies.
Until recently, we haven't addressed climate change in our conservation thinking, assuming species would always be where they are now and that parks and land-use protection could safeguard them. Now we know the problem is more complex. We need to protect species not only where they are, but also where they will be as the world warms. This requires siting of protected areas with change in mind. It means creating connective corridors between current populations of species and future safe havens. For instance, the Cederberg Wilderness in South Africa is an area that many climate models indicate will be an important destination for plants shifting range due to climate change. Connecting nearby current populations with this future stronghold will help secure these species both in the present and under future climates.
In some extreme cases, it may mean moving species to get them to where they need to be to survive.
The growing movements to pay countries and indigenous people to protect their tropical forests — known as REDD, for Reducing Emissions from Deforestation and Degradation — also holds enormous promise. It's a conservation strategy that not only prevents huge amounts of CO2 from being released into the atmosphere from forest destruction, but it also safeguards the astonishing biodiversity of tropical regions.
Climate change doesn't let us ignore all the pressing conservation problems that have concerned us for the past several decades. It simply ups the ante, forcing us to be smarter with limited resources, to ensure that our conservation gains of today are robust as climate changes in the coming decades.
As world leaders gather in Rio this summer, 20 years after the landmark Earth Summit, they should consider the intersection of two issues that made headlines at the original summit — climate change and biodiversity. Without attention to what climate change will mean for nature, the world may face functional and biological extinctions on land and sea, with far-reaching implications for the ecosystems of the world and the support they provide us.
Lee Hannah is senior researcher in climate change biology at Conservation International (CI), heading CI’s efforts to develop conservation responses to climate change. His research has appeared in journals including Nature, Conservation Biology and Bioscience. Hannah edited the book Climate Change and Biodiversity with Thomas E. Lovejoy. He authored the first undergraduate textbook on the biological impacts of climate change, Climate Change Biology. His latest book is Saving a Million Species: Extinction Risk from Climate Change.
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