Isn’t the climate always changing to some degree?
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Earth’s history is full of changes in climate in all magnitudes and time scales, but important aspects of the current warming are unusual. Current global temperatures are the highest they have been in at least the past 500 years, and perhaps the warmest in more than a millennium. Current concentrations of CO2 in our atmosphere are higher than at any other time in at least the past 500,000 years.
While past climate changes resulted from natural causes, people have been burning more and more fossil fuels over the last 150 years, and the resulting accumulation of greenhouse gases is increasingly affecting our climate. Most of the warming over the past half-century is attributable to human activities, according to the IPCC.
Another important difference: past climate changes typically unfolded over thousands or millions of years, in stark contrast to the current changes on scales of decades to a century. Thus, the rate of change, and not simply the fact of change, is important. Continued warming at current rates would be extremely unusual in geological terms. There is no scientific evidence that anything comparable to the rate of change projected in global temperature for the next century has occurred over the past 50 million years. That faster rate of change is an important factor in considering potential adaptation options for humans, plants, animals, and other species.
More information: National Climatic Data Center. “A Paleo Perspective on Global Warming“
“Surface Temperature Reconstructions for the Last 2,000 Years.” Committee on Surface Temperature Reconstructions for the Last 2,000 Years, National Research Council. National Academies Press.
IPCC Fourth Assessment Report: Working Group 1 Report, “The Physical Science Basis“