This answer focuses on classical measures of climate (e.g., temperature, precipitation, sea level, plus extreme events including floods, droughts, and storms), on other components of the Earth's climate system (e.g., greenhouse gases and aerosols, ecological systems), and on human health and socio-economic sectors. Climate change as defined in IPCC refers to statistically significant variations that persist for an extended period, typically decades or longer. It includes shifts in the frequency and magnitude of sporadic weather events as well as the slow continuous rise in global mean surface temperature. Thus the discussion here includes climate-weather variations on all temporal and spatial scales, ranging from brief-lived severe storms to seasonal El Niño events, decadal droughts, and century shifts in temperature and ice cover. Although short-term climate variations are considered predominantly natural at present, their impacts are discussed in this question because they represent a class of changes that may become more prevalent in a future climate perturbed by human activities (see Question 4). Attribution is used here as the process of establishing the most likely causes for the detected change with some defined level of confidence. The discussion includes both climate change that is attributable to human influence and climate change that may at present be natural but might in the future be modified through human influence (see Box 3-1).

Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate (see Table 2-1).

Concentrations of atmospheric greenhouse gases and their radiative forcings have generally increased over the 20th century as a result of human activities. Almost all greenhouse gases reached their highest recorded levels in the 1990s and continue to increase (see Figure 2-1). Atmospheric carbon dioxide (CO₂) and methane (CH₄) have varied substantially during glacial-interglacial cycles over the past 420,000 years, but even the largest of these earlier values are much less than their current atmospheric concentrations. In terms of radiative forcing by greenhouse gases emitted through human activity, CO₂ and CH₄ are the first and second most important, respectively. From the years 1750 to 2000, the concentration of CO₂ increased by 31±4%, and that of CH₄ rose by 151±25% (see Box 2-1and Figure 2-1). These rates of increase are unprecedented. Fossil-fuel burning released on average 5.4 Gt C yr^-1 during the 1980s, increasing to 6.3 Gt C yr^-1 during the 1990s. About three-quarters of the increase in atmospheric CO₂ during the 1990s was caused by fossil-fuel burning, with land-use change including deforestation responsible for the rest. Over the 19th and much of the 20th century the terrestrial biosphere has been a net source of atmospheric CO₂, but before the end of the 20th century it had become a net sink. The increase in CH₄ can be identified with emissions from energy use, livestock, rice agriculture, and landfills. Increases in the concentrations of other greenhouse gases -- particularly tropospheric ozone (O₃), the third most important -- are directly attributable to fossil-fuel combustion as well as other industrial and agricultural emissions.