A common skeptic argument is that climate has changed naturally in the past, long before SUVs and coal-fired power plants, so humans can't be the cause of the current global warming. Peer-reviewed research shows 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 highly 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.
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