An overwhelming scientific consensus agrees that global climate change is real and humans are causing it. The 2014 report of the Intergovernmental Panel on Climate Change (IPCC), a publication that aggregates current climate-science knowledge, concluded that recent global climate change has affected both human and natural systems on every continent and in the oceans . Global shifts in temperature, precipitation patterns, ocean levels, and the frequency of extreme weather events has impacted plant and animal populations as well as humans. The resulting extinctions, migrations, and behavioral changes will have catastrophic effects on entire ecosystems, fundamentally changing the world we inhabit.
These transformations are already happening. Global climate change has shifted the latitude and altitude of many species’ ranges, changed the timing and location of seasonal and migratory behaviors, altered the abundance of food and habitat, and transformed inter-species interactions . Current estimates suggest that for every million species, between 100 and 1000 go extinct each year (Figure 1). Many species remain undiscovered, so arriving at an exact number is difficult. Since the dawn of humanity, the rate of extinction has become 10,000 times greater. This rapid eradication of species has been called a sixth major mass extinction. Human-induced non-climate causes such as habitat destruction, hunting, and the introduction of invasive species contribute to this worrying total, but human-caused climate change plays a significant role, and its influence will only grow with time [2,3].
Figure 1: The Golden Toad (Bufo periglenes), which once inhabited the high altitude cloud forests of Monteverde, Costa Rica, is one of several species to go extinct directly as a result of climate change and was last observed in 1989 [2, 12].
Tampering with the Global Thermostat
Plants and animals have evolved to live in environments with particular temperature ranges, but those ranges are moving. Instead of being associated with particular latitudes, average temperatures are shifting towards the poles by approximately a quarter mile per year (3.8 feet per day). In order to survive changing temperatures, species must either relocate or evolve to living in warmer conditions . Species’ ability to respond depends on mobility, size, longevity, and lifecycle. For some, shifting global temperatures equates to a Red Queen’s race in which “it takes all the running you can do, to keep in the same place” . Although most plants are not predicted to keep up with even the most conservative models for changing global temperatures in the 21st century, most animals can adapt to slow rates of climate change. Very few animals besides large mammals, birds, and butterflies, however, will be able to keep pace with changing global temperatures if greenhouse gas emissions continue at their current rates . Even species that can outrun climate change may still not be safe if they rely on other, less mobile species within their native ecosystem. For instance, although primates are highly mobile, most are limited to a forest habitat that can’t keep up with environmental changes.
Organisms are already moving to keep up with the climate. A study of over 1,700 plant and animal species demonstrated that, since 1950, species have shifted on average either 3.8 miles towards the poles or 20 feet upward in elevation per decade in order to compensate for increasing global temperatures . A more recent study suggests species might actually be moving up to three times faster. On average, the 23 groups of species those researchers examined moved 10.5 miles towards the poles or 36 feet upward in elevation per decade . Marine species also moved to colder waters over the course of the last century (Figure 2) .
Figure 2: Between 1900 and 2010, almost all marine taxonomic groups have shifted to cooler water. This graph indicates the shift in geographical range to cooler or warmer waters, measured in km per decade, of various marine taxonomic groups. (Adapted from .)
Plant and animal species native to the poles and upper elevations are lost as their environments warm beyond their species’ tolerance; they have nowhere else to go. In Antarctica, penguin populations are declining, as are polar bear populations in the Arctic. Surviving bears are getting skinnier due to receding sea ice and a shorter hunting season .
Evolution can also be a costly response to climate change. Only a small subpopulation of a species is likely to possess the genetic variation required to survive in changing climatic conditions. Such a severe population bottleneck leads to a subsequent loss of genetic diversity and further inbreeding. As with the wooly mammoth, evolving in an attempt to adapt to climate change is likely only to lead to developmental and genetic defects and perhaps a temporary extension of a species’ ability to survive .
Temperature shifts, however, are not the sole cause of migration. Precipitation patterns altered by global climate change are also a critical factor in determining species range and can sometimes work to counteract the impact of warming temperatures . A study of 99 bird species in the Sierra Nevada Mountains could only explain the species’ twentieth-century migration patterns when accounting for both temperature and precipitation. Rising temperatures pushed some species to higher, cooler elevations, but increasing precipitation upslope pushed others downslope . As some species respond more to changes in temperature, others respond more to changes in precipitation, and still others are unable to respond at all. Thus, important interspecies relationships within ecosystems are destroyed.
The New Cycle of Life
Changing global temperatures also affect the timing of the occurrence of periodic lifecycle events. Across the second half of the twentieth century, seasonal events in animal and plant lifecycles occurred about 2.8 days earlier with each passing decade [1,6]. These changes, such as flowers blooming when pollinators are not active, mammals coming out of hibernation with less food available, or birds and butterflies migrating early can disrupt the timing of interspecies interactions . In the subalpine meadows of the Rocky Mountains, for example, the Columbian ground squirrel is emerging from hibernation 10 days later than it was 20 years ago due to late spring snowfalls. This delay causes a shorter summer eating period and a subsequent 20% decline in the adult female survival rate through the next hibernation period .
A Rising Toxic Tide
Rising water levels are causing coastal regions to shift inland. Species in coastal wetlands, seagrass meadows, mangrove forests, and salt marshes are squeezed between the encroaching ocean and spreading urban communities. Rising temperatures in coastal regions has also resulted in a decline in kelp forests . Kelp forests—large colonies of brown algae—provide essential food and protection for fish, invertebrates, birds, and mammals. Rising temperatures have also allowed massive harmful algae blooms to invade cooler waters, where they produce toxins that cause massive die-offs of fish and shellfish .
Marine ecosystems further from shore are also being affected. Fisheries are shifting poleward and decreasing in productivity. As global climate change worsens, fishery abundance and diversity is expected to decrease, particularly in the tropics and isolated seas. Marine abundance and diversity may increase at mid and high latitudes, but not enough to alter global declines. Warmer waters can’t hold as much oxygen, so warming oceans are projected to expand low-oxygen dead zones, areas that can’t support aquatic life.
As carbon dioxide is released into the atmosphere, some dissolves in the oceans, making them more acidic. This ocean acidification threatens coral reefs. In warm ocean waters, acidification results in coral bleaching (Figure 3) and mass death, which ultimately destroys whole reef ecosystems—some of the most diverse and fragile ecosystems on the planet . On the Great Barrier Reef, unusually hot temperatures in 1998 and 2006 resulted in coral bleaching in 50-60% of reefs. In both events, five percent of the reefs, which took thousands of years to develop, were permanently killed .
Figure 3: Coral bleaching from ocean acidification results in the subsequent loss of the entire reef ecosystem, which depends on the coral for survival. In the background, healthy coral is surrounded by fish. Bleached coral in the foreground is abandoned. This picture was taken off the Keppel Islands of the Great Barrier Reef in 2011 [11, 13].
Climate change doesn’t just impact humans: it poses a threat to most species on the planet. For their sake as well as for ours, it is more important than ever to mitigate the damaging effects of global climate change.
Matthew Schwartz is a fifth-year PhD student in the Biological and Biomedical Sciences program and the Department of Genetics at Harvard Medical School.
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