Linking climate and habitability

From the streets of New York City to the rivers in India to the glaciers in South America, humans are warming the planet by emitting more and more greenhouse gasses. In a study published in Nature last year, scientists for the first time linked the effects of climate change specifically to human activity.

"We're beginning to get the picture that climate change, influenced by humans, is beginning to influence ecosystems," says Cynthia Rosenzweig of NASA's Goddard Institute for Space Science (GISS), the study's lead author.

For astrobiologists, learning how the Earth responds to climate variations could be useful in better understanding planetary habitability. A planet's potential to develop and sustain life, as outlined by NASA's astrobiology program, depends on three primary factors.

The planet must have a reliable energy source, liquid water and appropriate conditions for the formation of complex organic molecules. Many of these characteristics are affected by the planet's atmosphere, which plays an important role by heating up or cooling down a planet.

Venus, for instance, might be habitable if not for its punishing greenhouse atmosphere. By linking climate change with human activity, the Nature study provides astrobiologists with clues about how life on a planet can affect habitability.

"Climate change in some ways is an analogy to different environments in space," says Rosenzweig. "It's really quite amazing. We've only had 0.74 degrees [Celsius change in global surface temperature during the past century], and so many systems are changing. So, while it really does speak to the potential for biological life in very different environments, it also shows how creeping shifts can have dramatic consequences."

With carbon dioxide being pumped into the atmosphere from the daily burning of fossil fuels, temperatures on Earth are now rising, and this rise in temperature is having a significant impact on physical and biological systems around the world. Glaciers and permafrost are melting, lakes and rivers are warming, flowers are blooming earlier, birds are migrating sooner, and both plant and animal species are searching for higher ground.

Changes in the natural ecosystems are also starting to impact humans directly, says the study's co-author David Karoly, professor of the School of Earth Science at the University of Melbourne in Australia.

For example, the early melting of snow packs in the western United States has a direct impact on summer time water resources. Earlier blooming affects the relationship between insects and flowering plants, and that can directly impact agricultural production.

The study not only verifies that climate change is occurring, it also clearly identifies the impacts of that warming and attributes them to human activity. In order to make the case, GISS researchers along with scientists from 10 other institutions developed a database of approximately 80 peer-reviewed papers of climate change research.

All studies had two things in common. They were long-term investigations that had data for at least 20 years between 1970 and 2004, and their findings showed a "statistically significant trend" in temperature-related changes.

Through these studies, the research team analyzed more than 29,000 data series of physical and biological systems. They identified the changes that were consistent with warming and, through statistical analyses, compared those changes to temperature trends around the world.

They found that human-induced increases in temperature account for 95 percent of observed changes in physical systems, such as glaciers, spring river runoff and warming of water bodies, and 90 percent of changes among plants and animals.