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By comparing historical records and modern measurements from weather stations and satellites, it has become clear that Earth's average temperature has generally been increasing since the 1970s. Studies suggest that the period from January 2000 to December 2009 was the warmest decade in the past 1,000 years. Since then, the planet's average temperature has remained very high by recent historical standards, and in 2016 Earth's surface temperature was the warmest since modern recordkeeping began in 1880 (Figure 3.18).

Furthermore, the planet's average surface temperature has risen about 1.1°C since the late 19th century and it is forecast to rise by at least another 1.4°C during the 21^st century.

Such global warming is generally attributed to an increase in greenhouse gases, particularly carbon dioxide and methane, which absorb heat and prevent it from escaping into space. Most of these gases have been added to the atmosphere by human activity, notably the clearing of forests by slash and burn farming, changes in agricultural land use, increased use of fossil fuels, and rampant urbanization/industrialization. Since about 1860, levels of atmospheric carbon dioxide have increased by almost 40% and methane levels have more than doubled.

The role of greenhouse gases in influencing Earth's temperature seems to be confirmed by measurements of air bubbles trapped in ancient ice cores. These indicate that on the ice age Earth of 20,000 years ago there was 50% less carbon dioxide and an even greater reduction in methane compared with current levels.

As the orbital geometry initiated warmer conditions, it seems that the warming was accelerated by the spread of plant cover and the release of carbon dioxide and methane. The minor warming by these gases also triggered a larger increase in water vapor, the most common greenhouse gas.¹⁰ In recent times, the process has been further accelerated by human activity.

Nevertheless, the warming trend since the end of the last glacial advance has been punctuated by some major fluctuations. Reconstructions of past climate show relatively warm conditions around the year 1000, which enabled the Vikings to settle in Greenland and even reach North America, followed by a relatively cold period, or “Little Ice Age,” from roughly 1500 to 1850.

It is also worth noting that there are remarkably strong correlations between measures of past solar activity and global temperature. For example, the Little Ice Age, which began around 1300 and lasted until 1870, coincided with a period when few or no sunspots were observed. Moreover, the rise in temperature over the past 100 years has occurred when the Sun increased its output to its highest levels in the last millennium.

Although radiant heat from the more active Sun is not enough to explain the rise in 20th century temperatures, it has been suggested that a change in solar activity can have important side‐effects. For example, an active Sun generates a more powerful magnetosphere. This provides a stronger shield from high‐energy cosmic radiation. Scientists theorize that these cosmic rays affect our climate by ionizing particles and gases in our atmosphere. The ionized molecules act as nucleation points for water droplets and lead to the formation of clouds. These clouds reflect sunlight back into space but also trap heat radiated from the ground.

Scientific views on climate change will continue to be revised as new data become available, but the present consensus is that human‐made CO₂ in the atmosphere is modifying the climate and is the main cause of global warming.