The continuing scientific process has resulted in an understanding of climate change that is increasingly robust and has withstood a multitude of challenges; in most respects, the evolving science confirms the broad conclusions made in previous decades by the preponderance of scientists. Many details and complexities, however, remain nebulous. The average global surface temperature has increased since the Industrial Revolution, by about 0.6 to 0.9 o Celsius (1.1 to 1.6 o Fahrenheit) from 1880 to today. The global climate of the past few decades is likely approaching, or has already passed, the warmest since the rise of human civilizations around 12,000 years ago. At a regional scale, such as for the United States, significant climate changes have also been measured. Likely Causes of Global Climate Change

The evidence is strong that the Earth's climate is changing; the forces thought to be driving observed climate changes are discussed below, including the evidence that human activities, particularly greenhouse gas emissions, have contributed a large influence on top of ongoing natural variability. In response to the climate change, other components of the Earth system may also adjust, resulting in feedbacks to the climate that can either amplify ( positive feedbacks ) or dampen ( negative feedbacks ) the initial change in climate. Greenhouse Gases

Greenhouse gas concentrations in the Earth's atmosphere have increased dramatically since the Industrial Revolution, with carbon dioxide growing from about 280 ppm in 1850 to about 380 ppm today. Over the past 150 years, CO 2 concentrations have increased globally by more than one-third, from about 280 ppm to current levels of about 380 ppm ( Figure 5 ). Several different lines of evidence discovered in the past decade have led a large majority of scientists to conclude that human-related greenhouse gas emissions have contributed substantially to the increase in global mean temperature and other climate changes observed since the 1970s, and probably over the past century. Most studies indicate, and experts generally agree, that growth of greenhouse gas forcing, if it continues unabated, will raise global average temperatures well above natural variability. Studies have found that future climate change will not be evenly distributed geographically or temporally: even if the global mean temperature were to change very little, regional climate changes could be dramatic because of the uneven distribution of forcings by different agents and the connectedness of regions within the climate system. Although almost all regions are expected to experience warming, some regions are projected to become wetter while others become drier. Thus, while most climate scientists conclude with high confidence that future climate change, forced by greenhouse gases, land use change, and natural factors, is probable, the magnitude, rapidity, and details of the changes are likely to remain unclear for many years, or even decades. For some areas and systems, these changes would be positive. Science cannot explain the recent patterns of climate change without including the influence of elevated greenhouse gas concentrations. Although many uncertainties remain concerning the future magnitude, rate, and other details of climate change, a preponderance of scientists conclude that greenhouse gases emitted by fossil fuel use and other human activities are virtually certain to induce global average warming by at least 1.8 o C (3.2 o F) over the 21 st Century—or as high as 4 o C (5.2 o F), and possibly more than 6 o C (more than 9 o F). Some scientists have found that estimated ultraviolet variance since 1915 correlates poorly with global average temperature. Feedbacks to clouds are among the least understood processes and account for large differences among climate model results.
Almost all scientists agree that the Earth's climate is changing, having warmed by 0.6 to 0.9o Celsius (1.1 to 1.6o Fahrenheit) since the Industrial Revolution. Science indicates that the Earth's global average temperature is now approaching, or possibly has passed, the warmest experienced since human civilizations began to flourish about 12,000 years ago. During the 20th Century, some areas became wetter while others experienced more drought. Most climate scientists conclude that humans have induced a large part of the climate change since the 1970s. Although natural forces such as solar irradiance and volcanoes contribute to variability, scientists cannot explain the climate changes of the past few decades without including the effects of elevated greenhouse gas (GHG) concentrations resulting from fossil fuel use, land clearing, and industrial and agricultural emissions. Over the past 150 years, measured carbon dioxide concentrations have risen by more than one-third, from about 280 parts per million (ppm) to about 380 ppm. The United States contributes almost one-fifth of net global greenhouse gas emissions. Some impacts of climate change are expected to be beneficial (e.g., increased agricultural productivity in some regions), whereas others are expected to be adverse (e.g., drought in some regions, rising sea levels in some coastal areas).
Forecasting future climate conditions is challenging, and some major processes remain poorly understood. However, methods are improving to characterize the risks. Scientists have found it is very likely that rising greenhouse gas concentrations, if they continue unabated, will raise the global average temperature above natural variability by at least 1.5o Celsius (2.7o Fahrenheit) during the 21st Century (above 1990 temperatures), with a small likelihood that the temperature rise may exceed 5oC (9oF). The projections thought most likely by many climate modelers are for a greenhouse gas-induced temperature rise of approximately 2.5 to 3.5oC (4.5 to 6.3oF) by 2100. However, the magnitude, rapidity, and details of the change are likely to remain unclear for some time. Future climate change may advance smoothly or sporadically, with some regions experiencing more fluctuations in temperature, precipitation, and frequency or intensity of extreme events than others. Some scientists emphasize potential beneficial effects of climate change, or count on the ability of humans to adapt their behaviors and technologies to manage climate change in the future; other scientists argue that the benefits of climate change may be limited, even accounting for probable adaptation and its costs, and that there are risks of abrupt, surprising change with accompanying dislocations.
The continuing scientific process has resulted in a better understanding of climate change and generally confirms the broad conclusions made in previous decades by the preponderance of scientists: that human activities emit greenhouse gases that influence the climate, with potentially serious effects. Details have been revised or refined, but the basic conclusion of the risks persists. The principal questions remaining for the majority of scientists concern not whether greenhouse gases will result in climate change, but the magnitude, speed, geographic details, and likelihood of surprises, and the appropriate timing and options involved in addressing the human components of climate change.