Isotopes are used to measure past climate properties. Deuterium and oxygen 18 are the most commonly used climate proxies. Lighter isotopes evaporate more readily from the ocean, so water vapor in the atmosphere is isotopically lighter than ocean water. This vapor gets lighter still as it is transported to higher latitudes while losing mass by precipitation. These processes leave an isotopic signal of temperature and continental ice volume in ice cores and deep sea sediment cores.

The current Holocene epoch is considered to be a time period of relatively stable climate compared to earlier geological periods. Still, some significant changes in temperature and sea level did occur. These climatic fluctuations include the Medieval Warm Period and the Little Ice Age, and more recently global warming. Temperature data for the 20^th century shows a strong warming from about 1970 to the present day, typically associated with anthropogenic forcing including greenhouse gas and aerosol emissions.

Climate sensitivity is defined as either the temperature change resulting from a doubling of atmospheric carbon dioxide concentration or the temperature change resulting from a 1W/m2 increase in radiative forcing. There are several different climate sensitivities that take into account different feedbacks in the climate system. The simplest climate sensitivity is black body sensitivity, which does not account for any feedbacks but gives the temperature change resulting just from a change in radiative forcing.

Renewable energy sources are discussed. These include wind energy, solar energy, biomass energy and geothermal energy. Energy from wind is acquired through the use of large wind turbines. These turbines ideally need to be located in areas where there is strong wind and low atmospheric turbulence. Solar power is collected using both photovoltaic solar cells and concentrated solar power. Energy from biomass can be produced in two ways: burning biomass to generate electricity or fermentation to produce fuel ethanol.

There are two ozone problems in the atmosphere. Tropospheric ozone in the form of photochemical smog is sometimes dangerously high whereas stratospheric ozone concentration is sometimes dangerously low. Photochemical smog is created through chemical reactions between UV radiation from the run and nitrogen oxides that are emitted from automobiles. High concentrations of tropospheric ozone are dangerous because of the damage ozone can cause to a person’s airway if it is inhaled. The EPA has specified limits of ozone concentration but several counties in the USA exceed these limits.