The Causes of Climate Change
Life as we know it — A natural blanket of greenhouse gases in the atmosphere keeps the planet warm enough for life as we know it — at a comfortable 15°C today. Human-caused emissions of greenhouse gases have made the blanket thicker, trapping heat and leading to a global warming. Fossil fuels are the single biggest source of human-generated greenhouse gas emissions.
Regulating earth's climate — The blanket of greenhouse gases that occurs naturally in the troposphere — representing less than one percent of the entire atmosphere — serves the vital function of regulating the planet’s climate. When solar energy in the form of visible light strikes the Earth, it warms the surface. Being much cooler than the sun, the Earth emits this energy back out to space in the form of infrared, or thermal, radiation. Greenhouse gases block the infrared radiation from escaping directly into space. The resulting "natural greenhouse effect" keeps the planet some 30°C warmer than it would otherwise be, which is essential for life as we know it.
Recent dramatic changes in the atmosphere — The Earth’s average temperature seems to have been remarkably stable for the past 10,000 years, varying by less than 1°C, allowing human civilization to thrive at what is today a comfortable 15°C. But the very success of our civilization risks disrupting the climate that has served us so well until now.
The problem we now face is that since the start of the industrial revolution some 250 years ago our emissions of greenhouse gases have been making this blanket thicker at an unprecedented speed. This has caused the most dramatic change in the atmosphere’s composition for at least 650,000 years. Unless we make significant efforts to reduce our emissions of greenhouse gases, the global climate will continue to warm rapidly over the coming decades and beyond.
The enhanced greenhouse effect — The reason these "artificial" emissions are such a problem is that, in the long term, the Earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate system must adjust somehow to restore the balance between incoming and outgoing energy. The result is known as the "enhanced greenhouse effect".
Complicated interconnections — The climate adjusts to the thicker blanket of greenhouse gases in large part through a "global warming" of the Earth's surface and lower atmosphere. This rise in temperature is accompanied by other changes, for example in cloud cover and wind patterns. Some of these changes may enhance the warming further (positive feedbacks), while others may counteract it (negative feedbacks). These various interactions complicate scientists’ efforts to determine precisely how the climate will change over the decades to come.
Greenhouse gas emissions — Fossil fuels formed by long-dead plants and animals are the single biggest source of humanity’s greenhouse gas emissions. Burning coal, oil and natural gas releases billions of tons of carbon every year that would otherwise have remained hidden in the Earth’s crust, as well as large amounts of methane and nitrous oxide. More carbon dioxide is released when trees are cut down and not replaced.
Meanwhile, massive herds of livestock emit methane, as do rice farms and waste dumps. The use of fertilizers produces nitrous oxide. Long-lived gases such as CFCs, HFCs and PFCs, used in air conditioning and refrigeration, are manufactured by industry and eventually enter the atmosphere. Many of these greenhouse gas-emitting activities are now essential to the global economy and form a fundamental part of modern life.
Assessing the Science: The Intergovernmental Panel on Climate Change (IPCC) — The United Nations, through the United Nations Environment Programme and the World Meteorological Organization, established the Intergovernmental Panel on Climate Change in 1988 to investigate and analyze the best published science on the issue. Since 1990 the IPCC has produced authoritative reports every five or six years assessing the state of the science through observations and forecasts of future trends.
How the IPCC works — The IPCC does not conduct new research, but rather, its mandate is to make policy-relevant assessments of the existing worldwide literature on the scientific, technical and socioeconomicaspects of climate change. The IPCC reports draw on the work of thousands of experts from all regions of the world. The Fourth Assessment Report came out during 2007, in four volumes, each prepared by a separate working group.
In preparing the reports, drafts are circulated to specialists with significant expertise and publications in the field. Their comments go back to the IPCC authors who in turn prepare a second review to governments and to all authors and expert reviewers. Governments and expert reviewers can provide comments restricted to the accuracy and completeness of the scientific/technical/socioeconomic content and the overall balance of the drafts. The final document reflects differing views that are supported either scientifically or technically.
Key findings and how to act on them — Each report has a Summary for Policymakers, approved line by line by the government delegations of IPCC member countries during a plenary session of the Working Group who produced it. Lead authors of the report are present, ready to explain the scientific facts supporting the statements contained in the Summary. Changes can only be made if there is agreement with the lead authors, to make sure that they are consistent with the underlying scientific and technical assessment. The Summary represents the point of agreement on the report’s key findings: participating governments acknowledge that there is enough scientific evidence worldwide to support the document’s statements.