Past Climate

greenhouse effect     climate past    climate present    climate future

What is the history of the earth's climate? By studying glaciers and ice sheets, scientists have been able to infer global temperatures over the past few hundred thousand years (1). 

The climate history is complex, with huge swings over long periods of time. Over the past few hundred thousand years (or perhaps few million years), the earth’s climate has oscillated between long cold unstable periods, known as ice ages, and short warm periods, known as interglacial periods.  There have been multiple ice ages, not just one (1,2).  These oscillations in climate are caused by complicated interactions between the earth’s orbit and the climate system (1). 

This graph shows estimated temperature from one ice sheets in Antarctica (Votosk station).  This graph reads backward in time, with today being on the far left, and 400,000 years ago being on the far right.  Zero on the y axis is relative to today's temperature, so -8 is 8 degrees C colder than today.

Note the short warm periods between long periods of highly variable and cold climate.  Also, note how comparatively stable the past 10,000 years have been as compared to the past 400,000 years. 

This historic instability of climate is perhaps the most alarming part of climate history. Indeed, ice sheets in Greenland show that the local climate can change by many degrees Celsius over just a few decades (1). It is suggested that these rapid shifts in climate are due to sudden changes of ocean and atmospheric circulation(1). Thus, one of the big worries with climate change is that we will somehow perturb the earth's climate system and cause a rapid (few decades) shift in climate.

By comparison, what does the modern climate look like? For the past 10,000 years, the climate has been warm and very stable.  During this time, human civilization has spread and covered the globe, and become accustomed to a stable climate. 

Here is a graph of the past 1,000 years of climate, from AD 1,000 to AD 2,000.  The different lines on the graph represent results from different data in different locations, with the experiment’s authors cited below the diagram.  Note that the y-axis is difference from the mean temperature.  For the past 1,000 years, the climate has stayed very constant, varying no more than half a degree C (1 degree F).  (Compare this variability with the long-term variability in the above chart).

Small changes in this climate over this period, though, were noticed to have a big effect. Notice that starting in the 1400s, and lasting until the 1800s, a few of these lines drop in temperature. This represents a period of slight cooling in the Northern Hemisphere, the result of which was significant effects on European civilization, adversly affecting agriculture (1), and even contributing to wiping out Viking colonies in Greenland (1).

In summary, past climate records provide a number of lessons for us. 1) The climate stability human civilizations have seen is unusual. 2) The climate, can change rapidly and, perhaps disastrously, and has done so many times in history. 3) Small changes in climate had real impacts on civilizations. In short, climate history shows that we have to respect the climate system, and not take it's services for granted.