Introduction
The scientific issue of global warming can be broken down into three main questions: Is global warming a reality? Are human activities causing climate change, and if so, to what extent? What is the forecast for the future?

Warming: Fact or Fiction?
The climate of the earth is indeed warming, with an increase of approximately 1 – 1½ degrees Fahrenheit in the past century. The warming has taken place as averaged globally and annually; significant regional and seasonal variations exist. The amount of warming aloft has been more difficult to quantify, but new research suggests a trend similar to that at the surface.

Impacts can already be seen, especially in places like Alaska - where melting of glaciers and the retreat of Arctic sea ice have increased the vulnerability of local populations to effects such as coastal erosion.

Human Influence
Determining to what extent the current warming is due to human activity is complicated because large and sometimes sudden climate changes have occurred throughout our planet's history - most of them before humans could possibly have been a factor. Furthermore, the sun/atmosphere/land/ocean "climate system" is extraordinarily complex. For these reasons it cannot be conclusively stated that humans are completely responsible for global warming.

However, it is known that burning of fossil fuels injects additional carbon dioxide and other so-called greenhouse gases into the atmosphere. This in turn increases the naturally occurring "greenhouse effect," a process in which our atmosphere keeps the earth's surface much warmer than it would otherwise be.

Computer model simulations and other assessments of the rate of global warming and the amount of increased greenhouse gases provide strong evidence that a significant portion of the current warming is a result of human activities.

Humans are also changing the climate on a more localized level. The replacement of vegetation by buildings and roads is causing temperature increases through what's known as the urban heat island effect. In addition, land use changes are affecting impacts from weather phenomena. For example, urbanization and deforestation can cause an increased tendency for flash floods and mudslides from heavy rain.

The Future
There is uncertainty in forecasting both the amount of warming and magnitude of resultant specific impacts. Potential outcomes during the next century and beyond range from moderate and manageable to extreme and catastrophic, depending on a number of factors including location and type of effect.

A better understanding of the climate system is vital, as are improved observations of that system and further advancements in the computer models used to help forecast the future state of our climate. However, the rate of climate change could exceed the pace at which scientists can provide irrefutable answers.

Variability is an ever-present part of our climate regardless of cause, and therefore the ability to adapt to any changes that do occur is crucial.