Greenhouse gas concentrations in the atmosphere are increasing due to the activities of man. We expect that this will act to warm the surface of the Earth by enhancing the greenhouse effect. However, it is difficult to make precise predictions of how much warming will occur and the associated climate changes that will follow. This is due to the complexity of the Earth's climate system. We employ computer models of climate to make predictions of climate changes. The models predict significant and perhaps harmful climate changes will come about in the future due to anthropogenic greenhouse gas emissions. However the models are uncertain and key questions still remain unanswered:
We will now take a closer look at the climatic history of the Earth in an attempt to address the first three questions listed above. We will first examine what we know about the historical temperature record to highlight the magnitude of past natural climate changes (prior to the Industrial Revolution). The long ago climate record is estimated based on indirect data and is thus uncertain, but significant past climate changes are obvious. Finally, we will examine the recent measured temperature changes and try to place them in some sort of historical perspective. This may help us to answer the question: Is the temperature change that we are seeing now unprecedented (and defintely caused by humans) or could it be largely the result of a natural fluctuation in global average temperature?
There is much evidence in historic geological records that says no!
Not only is the earth's climate always changing, but a mere 18,000 years ago the earth was in the grip of an Ice Age, with alpine glaciers extending down river valleys and huge ice sheets covering vast areas of North America and Europe.
Presently, glaciers cover only about 10% of the earth's land surface. Most of the ice is in the Greenland and Antarctic ice sheets. If global temperatures were to rise enough so that all of this ice melted, the level of the ocean would rise about 65 m (213 ft.). Although no one expects this to happen any time soon, keep in mind that through much of its history, the surface temperatures on Earth were much warmer than they are today, with little or no permanment glaciers.
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Instrumental weather records are available for only the last 200 or so years. To estimate the state of the Earth's climate further back in time, we must use proxy records, which are inferred records of past climate that are not based on direct measurements. Proxy records include archeological information about past human civilizations, written accounts, and environmental indicators.
Qualitative weather observations and indirect indicators, such as historical and archaeological data, allow decent reconstructions of climate in some parts of the world back to medieval times and more scant reconstructions back a few thousand years.
A wide range of scientific techniques can be applied to different kinds of environmental evidence to provide a picture of climatic variation. A few of these are:
These are collectively refered as environmental indicators. Keep in mind that these methods only provide estimates of past climatic conditions, they are by no means exact. The information is difficult to interpret and oftentimes incomplete. In general, the further back we go in time, the more uncertain the estimates of past climates.