Evolution or Extinction?

Associate Professor of Integrative Biology
University of Texas at Austin

Camille was a featured scientist in the Sally Ride Science publication Our Changing Climate: Ecosystems . For our blog, she would like to share her article that was published in Gincana 3, a publication of the Secretariat of the Convention on Biological Diversity, United Nations (2007).

Will global warming lead to the sixth mass extinction event? Or will life be more resilient? Will the teeming biodiversity that we now enjoy collapse down to a few, extremely hardy souls, or will evolution save the day? Climate change is a natural part of Earth's history, so why worry? Answers are even now upon us—both extinctions and adaptations are already happening, and both will continue to shape life as we know it over the coming centuries and millenia.

The study of impacts of climate change is not a new topic in biology. It has a rich history in the scientific literature, since long before there were political ramifications. Way back in 1917, a scientist named Grinnell concluded that the geographic boundaries of many species were determined by climate—individuals living at the edge of their range were living in as extreme an environment as the species could tolerate and survive. The history of biological research is full of studies of the impacts of weather and climate change on wild species. During the 1930s and 1940s, the climate in northern Europe “ameliorated,” bringing warm summers and mild winters. Researchers published a plethora of papers about earlier spring flowers and northward expansions of the ranges of birds and butterflies. In the 1960s and 1970s, European climatic conditions became “harsh”, with cold and wet summers starting about 1950. A second wave of papers came out documenting the lateness of spring flowers and the southward contractions of the same species of birds and butterflies that had earlier expanded northwards. Given the dynamic nature of Nature, it’s no wonder that the public is confused about whether or not to worry about global warming.

Why is human-driven warming any different from recent natural variation in climate, such as that experienced in northern Europe over the 20th century? The answer is simple. Natural warming and cooling trends have been like a lone car on a deserted highway not bothering to stay in lane: the wobbles back and forth have been relatively small, short term, and there's a strong tendency to move back to the middle. Global mean temperature has hardly changed in the past 10,000 years. But now we are changing from an earth with temperatures that fluctuate a bit to one that will be warming into the foreseeable future. Whether you imagine this as the car heading off onto a new, unexplored highway, or just going into the ditch—a major climatic shift is in progress. If we want to predict the impacts of human-caused change in global climate, our best clues can be found by looking, not at biological impacts of twentieth-century fluctuations but much further back in time to when climate truly did show shifts of the magnitude that we now anticipate.

If we look over the past few hundred thousand years, we see in the fossil records that the freezing and warming cycles of the Pleistocene glaciations caused massive relocation of plants and animals. Range shifts of thousands of kilometers were common as Earth went from a glacial age (when much of Europe and North America were covered in ice) to an interglacial age (as we’re in now). If we go even further back, to when Earth was much warmer than today (several million years ago), we see that many species did go extinct in the transition from this “hot” Earth to the Pleistocene “cool” Earth. Most species that were adapted to "hot" Earth are long gone. Species that survived are adapted to a relatively cool Earth. Human-driven global warming is taking us into a future which is warmer than it has been for thousands, and possibly for millions, of years—to an Earth that will lie outside the evolutionary experience of many plants and animals currently living. It will be no surprise if these species suffer high extinction rates.

Less than 10 years ago, as this information was sinking in, biologists were struggling to foresee the future. Which species would be most sensitive to global warming? How many species would go extinct? Would there be winners as well as losers? I was involved in several independent teams struggling with these questions—from conservation organizations like the IUCN to scientific panels like the IPCC—and the conclusions were remarkably similar. While no-one felt that predictions about particular species could be made, the consensus was that the species most affected by global warming would be those restricted to cold climate habitats, like Earth’s poles or mountain tops, and those able to tolerate only a narrow range of temperatures (e.g. tropical corals). Less than a decade later, those very predictions have been borne out.

Mountain species are following the climates to which they are adapted by shifting their ranges to higher elevations. However, for a population already at the top of its mountain, the preferred elevation now contains only sky. In many regions, high-elevation species are being pushed off their mountaintops. We see this in the American pika—an adorable little mammal well known to mountain back-packers for skittering along talus slopes carrying flowers in its mouth. We see it again in an icon for European naturalists—the Apollo butterfly­—whose translucent white wings with their bright red patches glide effortlessly between craggy mountains. The Apollo butterfly and the pika have lost many of their lowest populations and are gradually becoming confined to only the highest mountains.

What of the ultimate "cold-Earth" species—those whose habitat is actually floating sea ice? Surprisingly, none have gone completely extinct; but as sea ice declines, populations are declining in numbers, and their ranges are slowly contracting poleward. The Emperor and Adélie penguins have declined by 70% - 95% at their most equatorial populations in Antarctica (along the Palmer Penninsula) as sea ice has steadily shrunk or disappeared. But more worrying for their long-term future is that even some populations closest to the South Pole have declined. The Arctic has its own martyrs. There's an emerging debate as to whether the polar bear should be the first species to have its official cause of decline listed as “global warming” under the U.S. Endangered Species Act.

At the other end of spectrum, systems that we associate with hot beaches, bath-warm waters and cold drinks—species that we might think would be hot-adapted—are also suffering. Sixteen percent of tropical coral reefs worldwide were killed off by heat during the single extreme El Niño of 1997/1998. A coming threat is the increasing acidity of the oceans. The pH of tropical waters has already dropped from 8.2 to 8.1 as carbon dioxide is absorbed and converted to carbonic acid. As pH continues to drop, the ability of animals to construct hard shells will decline dramatically. Some coral biologists fear that “business as usual” projections could lead to tropical corals being unable to build and maintain reefs by 2050.

We're seeing impacts of current warming on every continent and in every ocean. We're also seeing very similar responses in very different types of organisms—from butterflies in Finland to fish in the North Sea, from foxes in Canada to trees in Sweden, from birds in Antarctica to starfish in Monterey Bay, California. Forty-percent of wild species are showing changes in their distributions - shifting their ranges north and south towards the poles and up mountains. An astonishing 62% are showing changes in their seasonal timing—spring is earlier and fall is later. Birds arriving for their spring migration, butterflies emerging from overwintering, trees leafing out after winter dormancy and the first blooms of flowers are all about two weeks earlier than they were 30 years ago across the northern hemisphere. My colleague, economist Gary Yohe, recognized that this was what economists would call a “globally coherent” signal of climate change impacts in natural systems across the world. This coherence—this systematic pattern—is important because it tells us that species and systems for which we don’t have any data are likely to be showing similar responses to those with detailed, long-term data. Globally, we estimated that half of all wild plants and animals have been affected by recent, human-driven climate change.

While geographic patterns of humans contracting different diseases is well-documented, the distributions in the wild of organisms that cause those diseases are often not well-studied. However, parasites that cause tropical diseases are not fundamentally different from other wild species. We therefore expect them to respond in the same way as more charismatic species for which we have better long-term data on their natural distributions. Just as tropical birds and butterflies have spread into Europe and the USA, we expect, then, that parasites and their vectors will extend their ranges from the tropics towards the poles, introducing human diseases as they invade new areas. In fact, human health is already being affected. For the year 2000, the WHO estimated that 6% of malaria infections, 7% of dengue fever cases and 2.4% of diarrhea could be attributed to climate change. This is principally due to increased frequency and intensity of flood events, which in turn have been linked to human-driven global warming. These numbers are likely to grow as these diseases expand their geographic ranges.

We can already see more severe effects of disease spread in the wild world, particularly for a group of amphibians appropriately named "harlequin " frogs. These bespeckled jewels of the clouds have served as poster-children for preservation of tropical cloud forests. Ironically, now that many sites have successfully been protected, global warming has crept from behind and staked its claim. Seventy-four species of cloud forest harlequin frogs have gone extinct in Central America. They aren't dying because they're too hot, but because climatic conditions have now become perfect for a deadly fungus. As this fungus invades the mountains, even some of the highest elevation species have been lost.

Some species, those with short generation times like insects, are showing genetic adaptation—evolution—in response to climate change. Unfortunately, these changes are small and unlikely to protect species from climate-caused extinction. Even though the frequencies of existing "hot-adapted" genotypes are increasing across populations in many species, truly new traits are not emerging. We are not seeing new mutations that would allow species to exist in climates outside their previous range of tolerance. In other words, species can play around with the genetic variation they already have, but evolving new, even “hotter” adapted genotypes, is a process that’s likely to be too slow to keep pace with rapid, human-driven climate change.

The good news is some northern-hemisphere species are able to move their ranges faster than we thought they could as climate warms at their northern range boundaries. These are species that already had a few individuals that were good at moving. The proportion of “movers” has increased at the range boundaries, and this local evolution has allowed these species to expand northward into new territories very rapidly. More good news is that some species that are adapted to a wide array of environments—globally common, or what we call weedy or urban species—will be most likely to persist.

So what will life on Earth look like over the lives of people now being born? There’s obviously a range of possibilities, depending on how policy-makers and the public as individuals decide to change our habits. What are the possible future worlds? Even the minimum projections - of another 1.8° C - are more than twice what we’ve already seen. All of the changes I talk about above have occurred with just 0.7° C warming. “Business as usual” projections are for another 4° C rise, with some models estimating over 6° C rise in global temperature. These higher projections represent a climate the Earth hasn't had long-term exposure to for several million years - outside the evolutionary history of much of life on earth now. Temperature changes of the that magnitude (> 6° C) in the past have often led to substantial extinction events, especially if the climate change was rapid. We need to implement major emission reductions now so that we keep future global warming down to those lower projections—down to “just” another 1.8° C. We can’t afford the worst case scenario either in terms of conservation of biodiversity, human health, or our economic stability.

Reprinted by permission of Secretariat of the Convention on Biological Diversity.