How do we know that people have affected the climate?

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Topic: People changing climate. Basics.
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How do we know that people have affected the climate?

Scientists have seriously discussed the risk of human-induced climate change for at least three decades. Many governments and non-scientists started worrying in the late 1980s, when the problem made headlines in newspapers and on TV around the world. Since that time, scientists have discovered much more about the causes of climate change.

In 2001, the Intergovernmental Panel on Climate Change (IPCC) (you can read more about IPCC in Unit 2), which is made up of experts from around the world, concluded that it was virtually certain that man-made emissions of greenhouse gases had contributed significantly to the observed climate change over the last 30 to 50 years.

What is this conclusion based on? It is founded on accurate and more comprehensive observations of climate change. Since the 1800's, weather stations around the world have measured air temperature using thermometers. Since the mid 1900's, the CO2 content of the atmosphere has also been measured. Since the 1970's, satellites have measured the intensity of the Sun entering the atmosphere and the amount of heat radiated back into outer space. By analyzing samples of ice taken from deep down in glaciers and samples of mud taken from lake bottoms, scientists can also map climate change far back in time.

shelter when drilling for ice in Antarctica

1. DRILLING FOR INFORMATION: Outside it is colder than -40°C, so it is best to build a shelter when drilling for ice in Antarctica! Scientists from several European countries have been drilling for years and have gone more than 3 kilometers down into the ice. The oldest ice they have sampled is more than 900,000 years old. Photo: Marzena Kaczmarska/NPI

These observations have led scientists to try to find the causes of changes in the Earth’s climate and to predict how the climate will change in the future. Because the climate system is so large and complex, scientists have relied heavily on computer based climate models.

Climate models

A climate model is usually a computer program where scientists enter what they know about how sunlight, greenhouse gases, the atmosphere and the oceans interact and how they shape the climate throughout the world. The model can be used to study what the causes of an observed change in the climate might be. When scientists include both the impact of natural factors and man-made emissions of greenhouse gases and particles over the last hundred years, the model calculates a change in climate that closely resembles the change that has been actually observed.

model of climate and Kossos temple

2. MODELS: A model is a simplified representation of reality. What all models have in common is that they simplify what they are supposed to represent or describe. Here you see a model of a molecule (DNA, which contains our genes), and a model of a building (the Knossos temple on Crete).

Climate models can also predict how the climate will change in the future. For example, they can estimate future changes in temperature on the basis of assumed concentrations of greenhouse gases, particle concentrations, solar intensity and other conditions that affect the climate. The most complicated climate models require enormous computers and often take months to complete a single simulation.

The model estimates always carry some degree of uncertainty. Not only do models simplify reality, but there are also some mechanisms in the climate system that we currently do not understand well enough to put into models accurately. For example, scientists are still uncertain of the roles that particles, cloud cover and the oceans play in global warming. This is one reason why the climate models are unable to predict the future climate with full certainty. Another important reason is that we can only guess what future emissions of greenhouse gases and particles will be.

Below are some links that describe some of the reasons why model estimates are uncertain:

The more specific a forecast we make, the more uncertain it is. For example, it is much more difficult to make predictions about climate change in a specific country than it is to predict general developments for the planet as a whole. Likewise, a model may not be able to predict the exact year changes will take place, but it can state with more certainty a time period over which these changes are likely. Although we have a hard time knowing just how serious human-induced climate changes will be, exactly where and when the changes will take place, or what the consequences will be, we can be virtually certain that people have changed the climate and will continue to do so in the years ahead.

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About this page:

Author: Camilla Schreiner - CICERO (Centre for International Climate and Environmental Research - Oslo) - Norway.
Scientific reviewers: Andreas Tjernshaugen - CICERO, Norway - 2004-01-20 and Dr. Knut Alfsen - Statistics Norway, Norway - 2003-09-12.
Educational reviewer: Nina Arnesen - Marienlyst School, Oslo, Norway - 2004-03-10.
Last update: 2004-03-27.

Last modified: Friday, 17 May 2019, 3:22 PM