Brief Review and a Look Ahead

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 future climate changes due to the complexity of the climate system. Therefore, we rely on computer models of climate to make predictions of climate changes. The models are far from perfect, but they are the best tools we have to estimate how the Earth's climate may change due to increasing concentrations of greenhouse gases. All current models predict warming of the global average surface temperature. This is widely accepted by the scientific community. Unfortunately, the details of climate changes on regional scales and their potential impacts upon human populations and ecosystems remain uncertain.

Thus far, we have stated that the Earth's climate has changed in the past and listed several possible causes for climate changes. Today we will take a closer look at the climatic history of the Earth. We will first examine what we know about the historical temperature record to highlight the magnitude of past climate changes. Finally, we will examine the recent measured temperature changes in an attempt to answer the questions: Is global warming occuring right now? And if so, what is the likelihood that the warming is related to the recent increases in greenhouse gases?

Reconstructing Past Climates

Were the climates of Earth always as they are today?

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 a cold spell, 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.

Why do we need to understand the climates of the past?

• to give us some idea about the magnitude and speed of natural climate changes throughout the history of the Earth;
• to help us to understand present-day climate and how it might change in the future;
• to test hypotheses about the causes of climate change.

How do we know what climate was like in the past?

Please take a look at the three links provided in this section. There is probably no need to print those pages though.

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:

• Plant and animal remains (fossil records)
• Glacier fluctuations (advance and retreat of glaciers)
• Fluctuations in lake levels
• Dendroclimatology (tree rings)
• Oxygen isotope data
• Ice cores
• Ocean/lake sediment cores

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.