2.

2.1
2.2
2.3

2.1

2.1 Are human activities modifying the atmosphere?

Changes in climate are the result of both internal variability within the climate system and external factors (both natural and anthropogenic).Human emissions are significantly modifying the concentrations of some gases in the atmosphere (see historical record in Figure 2). Some of these gases are expected to affect the climate by changing the earth's radiative balance, measured in terms of radiative forcing (see estimates in Figure 3):

Greenhouse gases, which have a global effect, tend to warm the earth surface by absorbing some of the infrared radiation it emits.

The principal anthropogenic greenhouse gas is carbon dioxide (CO2),whose concentration has increased by 31% since 1750 to a level which is likely7 to have not been exceeded for 20 million years. This increase is predominantly due to fossil fuel burning, but also to land-use change, especially deforestation. The other significant anthropogenic greenhouse gases are methane (CH4) (151% increase since 1750, 1/3 of CO2's radiative forcing), halocarbons such as CFCs and their substitutes (100% anthropogenic, 1/4 of CO2's radiative forcing) and nitrous oxide (N2O) (17% increase since 1750, 1/10 of CO2's radiative forcing).

anthropogenic aerosols, which have a regional effect, are short-lived and mostly tend to cool the earth down.

known natural factors such as changes in solar irradiance and volcanic eruptions are expected to have only made small contributions to radiative forcing over the past century.

2.2

2.2 How well is climate change understood?

Complex computer models are used to predict future climate. Understanding of climate processes and their incorporation in computer models has improved. Although such models still cannot simulate all aspects of climate, confidence in their ability to provide useful projections has increased. They can now better reproduce the 20th century global warming , using both natural and anthropogenic forcing (see Figure 4).

However further research is required to improve the ability to detect, attribute and understand climate change, to reduce uncertainties and to project future climate changes.

2.3

2.3 To what extent is climate change due to human activities?

In light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years (i.e., about half of the warming over the last 120 years) is likely7 to have been due to the increase in greenhouse gas concentrations.

The warming over the past 100 years is very unlikely7 to be due to internal variability alone and is unlikely7 to be entirely natural in origin.