Chapter 2: Causes of change

The second chapter deals with the causes of these changes. It begins by mentioning that the Earth's energy budget is changed by increases in GHGs and aerosols, land use change (deforestation) and variations in solar radiation. However, the main focus in not on variations in solar radiation but rather on anthropogenic GHG emissions. It is hard to believe that emissions of GHGs have increase by 70% since only 1970:

"Global GHG emissions due to human activities have

grown since pre-industrial times, with an increase of

70% between 1970 and 2004"

The report goes on to say there is 'very high confidence' that human activity has caused the planet to warm. This is primarily due to the burning of fossil fuels (56% of total GHG emissions in terms of CO2 equivalent) and agricultural activity which releases CO2 and CH4. Connected with this is land use change which is also a big contributor. The result is alarming; that concentrations of CO2 and CH4 in the atmosphere are far higher than any time over the last 650,000 years.

A pie chart illustrates the contribution of different sectors in total anthropogenic GHG emissions in 2004 (in terms of CO2 equivalent). The largest contributor is energy supply (25.9%), industrial activity is second largest (19.4%), forestry and deforestation is third (17.4%), agriculture is fourth with 13.5%, fifth is transport with 13.1%, sixth is residential and commercial buildings with 7.9% and last is waste and wastewater with 2.8%.

Interestingly, the report mentions that if only natural forcing is accounted for by itself, (i.e., volcanic eruptions, Earth's orbit around the sun, and solar output), over the last 50years the Earth would have actually cooled. Models prove this; "Observed patterns of warming and their changes are simulated only by models that include anthropogenic forcings".

This section ends by pointing out that in addition to causing a temperature increase, human activities have had other impacts on the climate system. These are listed below:

• very likely contributed to sea level rise during the latter half of the 20th century
• likely contributed to changes in wind patterns, affecting extra-tropical storm tracks and temperature patterns
• likely increased temperatures of extreme hot nights, cold nights and cold days
• more likely than not, increased risk of heat waves, areas affected by drought since the 1970s and frequency of heavy precipitation events.

To further support the anthropogenic nature of these changes, the paper reports that the physical and biological changes that are occurring are 'very unlikely' to be solely due to natural variability.