Climate Variability

Climate variability refers to variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all spatial and temporal scales beyond that of individual weather events. Because of the complex and dynamic nature of climate, modeling studies are used to integrate several data sources to effectively integrate current knowledge about climate elements. These models are also essential to predicting future climate change and estimating effects of greenhouse gases on global climate. ¹

Causes of climate variability are known as climate forcing. Variability may be due to natural internal processes within the climate system (internal variability), or to variations in natural or anthropogenic external forcing (external variability). These include such processes as variations in solar radiation, deviations in the Earth's orbit, mountain-building and continental drift, and changes in greenhouse gas concentrations. There are a variety of climate change feedbacks that can either amplify or diminish the initial forcing. Some parts of the climate system, such as the oceans and ice caps, respond slowly in reaction to climate forcing because of their large mass. ²

Naturally Occurring Climate Variability

Perhaps the most well known naturally occurring climate variability phenomenon is known as the El Niño-Southern Oscillation (ENSO), an interaction between the ocean and the atmosphere over the tropical Pacific Ocean that has important consequences for weather around the globe. The ENSO cycle is characterized by coherent and strong variations in sea-surface temperatures, rainfall, air pressure, and atmospheric circulation across the equatorial Pacific. ³
The energy input to Earth from the Sun can vary, for example because of changes in Earth's orbital path, changes in the tilt angle of the Earth, or variations in the energy coming from the Sun. These changes occur over long time scales and lead to the cycling of Ice Ages and interglacial periods that have occurred over the last 500,000 years. An additional factor which can cause climatic fluctuations on yearly to decadal timescales is volcanic activity. Volcanic eruptions lead to enhancements of stratospheric and tropospheric aerosols which for the most part reflect solar radiation, hence leading to global cooling on a global average. However, regionally these can lead to warmer conditions by changing atmospheric circulation patterns, especially in the wintertime. ⁴ ⁵

Anthropogenic External Forcing

There is strong observational evidence from all continents and most oceans that natural systems are being affected by climate change, especially increases in temperature. The IPCC has estimated a combined net forcing of all human-related pollutants and concluded that it is extremely likely that humans have exerted a substantial warming effect on the climate.⁶ The net forcing by anthropogenic sources is composed of warming and cooling forces. The largest positive (warming) effect is by long lived greenhouse gasses especially carbon dioxide. This effect is in part masked by the negative (cooling) effect from sunlight scattered by particle pollution and other indirect effects such as increased cloud reflectivity. Methane (CH4) and Nitrous Oxide (N2O) are two of the major greenhouse gasses with forcing effects on climate after carbon dioxide. Other pollutants such as black carbon also exert a forcing effect on climate, one especially strong in certain regions such as Asia and the Arctic. ⁷