Ice core records show that atmospheric CO2 concentration varied in the range of 180 to 300 ppm over the glacial-interglacial cycles of the last 650 000 years. The ice core records also indicate that greenhouse gases co-varied with Antarctic temperature over glacial-interglacial cycles, suggesting a close link between natural atmospheric greenhouse gas variations and atmospheric temperature (IPCC, 2007).
For about a thousand years before the Industrial Revolution, the amount of most important long-lived greenhouse gases (CO2, CH4 and N2O) in the atmosphere remained relatively constant. However, since the Industrial Revolution, combustion of fossil fuels, agricultural activities and land use have caused continuous increases in greenhouse gases over about the last 250 years. The rates of increase in levels of these gases are dramatic about from 1850.
Yearly average background concentration of carbon dioxide (CO2) has increased by almost 40% since pre-industrial times from about 280 ppm to about 390 ppm (in 2010), and is still increasing at an unprecedented rate of on average 0.4% per year (Figure 14.2). However, atmospheric CO2 concentration increased by only 20 ppm over the 8000 years prior to industrialisation (IPCC, 2007).
The nitrous oxide (N2O) background concentration in 2010 was 324 ppb, almost 20% higher than its pre-industrial value. Based on ice core data the atmospheric concentration of N2O varied by less than about 10 ppb for 11 500 years before the beginning of the industrial era (IPCC, 2007). Since then, N2O concentration has continuously increased due to the increasing anthropogenic emission from agricultural activities and land use change. Over the past few decades, the N2O concentration has increased approximately linearly by about 0.8 ppb year–1 (Figure 14.3).
Atmospheric methane (CH4) concentration has increased dramatically since 1750 due the increasing anthropogenic emissions. Methane concentrations varied slowly between 580 and 730 ppb over the last 10 000 years, but increased by more than 1 000 ppb in the last two centuries, representing its fastest change over at least the last 80 000 years (IPCC, 2007). Current atmospheric level of methane is about 1800 ppb. However, in the last few decades, the rate of increase has slowed (Figure 14.4). This recent decline in growth rate implies that CH4 emissions have become comparable to removals due primarily to oxidation by the hydroxyl radical (OH).
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are emitted into the atmosphere only during anthropogenic emissions from industrial sources. Their atmospheric concentrations have only been detected since 1950s. However, as these gases play important role in stratospheric ozone depletion, their emission reduction strategies were declared in the Montreal Protocol (signed in 1987) for the protection of the ozone layer. Due to the effective reduction of their emissions, atmospheric concentrations are now decreasing by natural removal processes (Figure 14.5).
Tropospheric ozone (O3) is a short-lived greenhouse gas produced by chemical reactions of its precursor species (see Chapter 8). The background atmospheric concentration of ozone increased continuously in the last decades of 20th century (Vingarzan, 2004). Spatial and temporal distributions of ozone concentrations, however, show large variability and related to the concentration of the nitrogen oxides (NO and NO2), carbon monoxide (CO) and other precursor compounds.