What does past climate change tell us about global warming?

What The Science Says:
Climate reacts to whatever forces it to change at the time; humans are now the dominant forcing.

Climate Myth: Climate's changed before
Climate is always changing. We have had ice ages and warmer periods when alligators were found in Spitzbergen. Ice ages have occurred in a hundred thousand year cycle for the last 700 thousand years, and there have been previous periods that appear to have been warmer than the present despite CO2 levels being lower than they are now. More recently, we have had the medieval warm period and the little ice age. (Richard Lindzen)

A common skeptic argument is that climate has changed naturally in the past, long before SUVs and coal-fired power plants, and this somehow tells us that humans can't be the main cause of the current global warming. Peer-reviewed research and simple logic show this is not the case.

It's important to know there are a number of different forces acting on the Earth’s climate. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, they emit particles into the atmosphere which reflect sunlight, and the planet cools. When there are more greenhouse gases in the atmosphere, the planet warms. It's worth remembering that without some greenhouse gas  the Earth would be a ball of ice.

These forces are called "forcings" because they force changes in the global average temperature.

Looking at the past gives us insight into how our climate responds to such forcings. Using ice cores, for instance, we can work out past temperature changes, the level of solar activity, and the amount of greenhouse gases and volcanic dust in the atmosphere. Looking at many different periods and timescales including many thousands of years ago we've learned that when the Earth gains heat, glaciers and sea ice melt resulting in a positive feedbacks that amplify the warming. There are other positive feedbacks as well and this is why the planet has experienced such dramatic changes in temperature in the past.

In summary the past reveals our climate is sensitive to small changes in heat. 

What does that mean for today? Over the past 150 years greenhouse gas levels have increased 40 percent mainly from burning of fossil fuels. This additional "forcing" is warming the planet more than it has in thousands of years. From Earth's history, we know that positive feedbacks will amplify this additional warming.

The Earth's climate has changed in the past and ice cores and other measures tell us why. Based on this knowledge, and other types of evidence we know the human emissions of greenhouse gases are warming the climate.

The 'climate changed naturally in the past' argument is a logical fallacy known as non sequitur, in which the conclusion doesn't follow from the arguments.  It's equivalent to seeing a dead body with a knife sticking out the back, then arguing the death must be natural because people died naturally in the past.  It fails to even consider the available evidence.


Creative Commons License The Skeptical Science website by Skeptical Science is licensed under a Creative Commons Attribution 3.0 Unported License.