Have greenhouse gas emissions caused global temperatures to rise?
Greenhouse gases have continued to increase in the atmosphere. This is due largely to human activities, mostly fossil fuel use, land-use change, and agriculture. About 63% of the warming effect of greenhouse gas increases over the last 200 years is due to carbon dioxide.
The second most important greenhouse gas produced by human activities is methane which accounts for about 18% of the increased warming. (This is an important aspect of New Zealand's greenhouse gas emissions since sheep and cows produce methane).
The growth in atmospheric CO2 continues unabated and at an accelerating pace, in line with an accelerated consumption of fossil fuels. In contrast, while methane levels in the atmosphere continued to grow through the 1980s and 1990s, the growth rate steadily fell, resulting in methane levelling off from about 2000 at 2.5 times its level of only 300 years ago. However, measurements by NIWA show signs that methane is again growing.
Warming by greenhouse gases is offset in some regions by a cooling due to small airborne particles generated by burning fuel. These are concentrated around areas of industrial activity in the Northern Hemisphere and in developing countries. (The cooling effect of aerosols over the New Zealand region is expected to be small).
Global mean surface temperature has increased by between 0.3 and 0.6 °C since the late 19th century, a change which is unlikely to be entirely natural in origin. The balance of evidence suggests a discernible human influence on global climate. Much of the 10–25 cm rise in global average sea level over the past 100 years may be related to the rise in global temperature.