Global Warming

Introduction

Global warming is no longer just a prediction— it is actually happening. It is undisputed that the average temperature at the surface of the Earth has increased over the past century by about 1°F (0.6°C), with both the air and the oceans warming. Since 1880, when people in many locations first began to keep temperature records, the 25 warmest years have all occurred within the last 28 years. Scientists know with absolute certainty that the observed dramatic increase in the atmospheric concentrations of greenhouse gases since pre-industrial times (to levels higher than at any other time in at least the last 420,000 years) has been caused by human activities, mostly the burning of fossil fuels (coal, oil, and natural gas), and to a lesser extent, deforestation. The ability of greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, to trap heat at the Earth's surface is also scientifically well understood. While it has not yet been precisely determined how much of the recent warming was caused by human activities, the consensus among climate scientists is that most of the warming over the past 50 years was probably caused by human-produced greenhouse gases.



Extreme Events

It is difficult to blame a particular extreme weather event (such as a heat wave, flood-producing storm, or drought) on global warming, since weather fluctuates naturally. However, climate theory and models predict that global warming will increase the frequency and/or intensity of some types of extreme events. A warmed world, in other words, is like a loaded die that comes up “Heat Wave” or “Intense Rainstorm” more often than an unperturbed world. Thus, scientists can estimate the fraction of the risk of a particular kind of event for which humans are responsible. This section of the report presents examples of extreme events observed during recent decades that are consistent with the effects of global warming and may be harbingers of greater changes to come if we allow global warming to continue unabated.

In the United States, Chicago experienced one of the worst weather-related disasters in Illinois history when a heat wave resulted in 525 deaths during a 5-day period in July of 1995. The 106°F (41°C) reading on July 13 set a record for the warmest July temperature since measurements began. The combination of high humidity and high nighttime temperatures meant little respite from the heat could be found, especially for such vulnerable populations as the elderly and people with low incomes. Studies indicate that the increase in death rate from more frequent heat waves in the future will probably outweigh any decrease in death rate due to less severe winter cold in the U.S., because cold-related mortality is more complex and not so strongly tied to the severity of the cold.

Forests and wildfires

Scientists expect global warming to contribute to an increase in wildfire in several ways. As discussed in the previous section, droughts are expected to become more common and severe in some regions. Desiccating heat and lack of precipitation create ideal conditions for major ...