"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.

The researchers acknowledge that being able to conclusively say human activity is responsible for climate change was a challenge. They had to rule out the possibility that other factors, such as land use or natural variations in climate, could be causing these changes.

The research team was able to disregard these factors because they found very little evidence from the studies - less than one percent of the data series contained effects that were likely to be caused by something other than climate change.

One reason for the lack of evidence, according to Rosenzweig, has to do with methodological constraints. For example, many plant studies in Europe are carried out in protected areas, botanical gardens for instance, so factors such as land use are never a problem.

The report's findings are strongest in North America, Asia and Europe. What the study lacks is adequate data from the developing world, mainly because studies with long-term monitoring are hard to find in those regions. Rosenzweig says inadequate funding is a key issue but there are also inherent difficulties in studying those environments. Developing countries are predominantly located in tropical and subtropical regions.

"The systems there are also driven by moisture-driven seasons rather than temperature-driven seasons," says Rosenzweig.

So instead of summers and winters, there are wet seasons and dry seasons. And with very little variation in temperatures year round, measuring and quantifying warming patters is difficult since the warming is so ubiquitous.

"The impacts that are easiest to see are the ones that are most directly related to temperature change,. because the largest signals that are related to climate change are temperature changes," says Karoly. Those temperature changes have been larger in higher latitudes than in lower latitudes away from the equator."

The findings of this study are particularly significant because scientists analyzed impacts on a continental scale.

"We are engaged in the most significant environmental challenge that human beings have faced," Rosenzweig says. "While proving things on the global scale is important, as these scales become finer and finer, climate change becomes more and more real. The people in the continents are the ones who are going to have to respond."

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