This is a popular question that we have received more than a dozen times in various forms. And it's a fair question: If the Earth has cycled naturally between extremes larger than where we are now and even where we could be heading, why should someone believe that humans are causing the recent warming?
The other part of this view point is if the Earth reaches these extremes periodically on its own, why should people worry if it does so because of human actions?
The quick answer to the first part of the question, which is likely already known by most who pose the question in the first place, is yes: The Earth's climate has changed, many times, going back millions of years.
These large-scale climate shifts consist of colder ice ages followed by much warmer interglacial periods, characterized by melting ice sheets and higher sea levels.
The causes of these shifts were indeed natural. After all, humans didn't begin to significantly interfere with the climate system through the burning of fossil fuels until the Industrial Revolution.
Our distant past helps us see our future
So how can we say that the recent warming we have seen is caused by humans and not just this natural cycle?
By knowing about these natural cycles and studying them, it makes us even more confident that the recent warming we have seen over the past 150 or so years is caused by humans and our emission of greenhouse gasses into the atmosphere.
Scientists understand the natural processes behind the previous warm and cold periods that lead to ice ages. They occur in regular patterns called Milankovitch cycles. These cycles occur because the Earth's orbit around the sun is not constant.
The shape of the orbit changes, the tilt of the Earth on its axis changes and the even the direction of the axis changes over time. All of these changes result in varying amounts of energy (i.e. heat) that the planet receives from the sun. That, of course, determines how warm or cold the planet becomes.
While these cycles do impact Earth in much the same way as we are seeing now, they happen very, very slowly.
These cycles take place on 100,000 year time frames, and the amount of warming we have seen, even though it is "only" about 1.5°F (0.85° C) since 1880, would take many thousands of years to occur if the process were occurring purely naturally. Also, when you plot these orbital cycles out, we should be in a "cooling" phase of the cycle -- not warming.
The other important fact we learn when looking at these long-term cycles is that greenhouse gases, namely carbon dioxide and methane, move up and down with the global temperature. When greenhouse gases are high, the Earth is warm, when they are low, the Earth is cool.
But again, when these changes are occurring naturally, they take thousands to tens of thousands of years to occur, whereas humans have caused this to occur in just over 100 years. In fact, humans have pushed the level of carbon dioxide in our atmosphere to levels not seen in millions of years. The last time levels were this high, sea levels were several meters higher and temperatures were several degrees warmer.
So yes, the climate changes naturally, in much the same way it is changing now, but it happens much, much slower. That in turn gives the Earth, and its various life forms, time to adapt. When these changes occur rapidly, you can have mass extinctions.