Is global warming really a threat?

Absolutely, respond most scientists, but they have only recently been able to approach a basic agreement about our changing climate.

First, the Earth has gotten warmer. Since 1850, average global temperatures have risen about .6 degres Celcius. The most likely culprits are the greenhouse gases, such as carbondioxide, released by humans burning fossil fuels, and clearly of lands. Sera levels have also risen about 4 to 8 inches during the past century.

Second, the concentration of greenhouse gases (or GHG) in the atmosphere is near its highest point in recorded history. Since the Industrial Revolution, concentrations of carbon dioxide have risen 30%.

Based on studies of air trapped in ancient ice, today's level are higher than any time in the last 420000 years. If GHG concentrations rise, as expected, concentrations could cross a dangerous threshold, although that designation is contentious.

Finally, almost every scientist agrees upon one thing that the future is highly uncertain.

Projections state a rise of 1.4 to 5.8 degree Celcius by 2100. Other scientific concensus shows cracks beyond this point.

The Earth system has more unknowns that we are generally willing to acknowledge

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The United Nations establishes the Intergovernmental Panel on Climate Change in 1988. Carbon dioxide concentrations in the atmosphere reach 350 parts per million from the pre-industrial level of 280 ppm. Carbon dioxide levels above 500 ppm are thought to pose a "dangerous" level of interference with the climate system, according to the United Nations.

In 1990, the IPCC presents its First Assessment Report, stating that human activities are substantially increasing the concentrations of greenhouse gases in the atmosphere. Two years later, the United States, along with more than 100 other countries, sign the United Nations Framework Convention on Climate Change in Rio de Janeiro, Brazil. It recognizes the climate system as a shared resource and launches efforts to curb climate change.

The Kyoto Protocol is negotiated to reduce greenhouse gas emissions 5.2 percent below 1990 levels by 2012.

The U.S. National Academy of Sciences publishes a report in 2001. The panel declares "temperatures are, in fact, rising" and human activity is the likely culprit, although it does not define the influence of natural climate variability. Predictions of a 3 degree Celsius warming are called "consistent" with climate science.

The Kyoto Protocol enters into force on February 16. The treaty was ratified by more than 140 countries. Concentration of carbon dioxide now stands at 372 parts per million, higher than at any time in at least the past 420,000 years, according to David King, chief science adviser to the British government.

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This is a matter of great concern. I'm worried.