Climate through human history

Present-Day Climate Variability

To understand some of the reasons for the year-to-year variability which occurs under present conditions it is worth focusing on two different types of influence: volcanic eruptions, and the El Niño/Southern Oscillation phenomenon.

Volcanic Eruptions

In the last century, two significant climate modifying eruptions have occurred. El Chichón in Mexico erupted in April of 1982, and Mount Pinatubo went off in the Philippines during June, 1991.

Eruption of Mount St. Helens on May 18, 1980 which had a local effect on climate because of ash reducing the reception of solar radiation on the Earth's surface. Mount St. Helens had very minimal global effect on the climate because the eruption occurred at an oblique angle putting little sulfur dioxide into the stratosphere. (Source: U.S. Geological Survey, photograph by Austin Post).

Of these two volcanic events, Mount Pinatubo had a greater effect on the Earth's climate and ejected about 20 million tons of sulfur dioxide into the stratosphere. Researchers believe that the Pinatubo eruption was primarily responsible for the 0.8 degree Celsius drop in global average air temperature in 1992. At first, scientists thought that the dust emitted into the atmosphere from large volcanic eruptions was responsible for the cooling by partially blocking the transmission of solar radiation to the Earth's surface. Recent stratospheric data suggests that large explosive volcanic eruptions also eject large quantities of sulfur dioxide gas which remains in the atmosphere for as long as three years. Atmospheric chemists have determined that the ejected sulfur dioxide gas reacts with water vapor commonly found in the stratosphere to form a dense optically bright haze layer that reduces the atmospheric transmission of some of the sun's incoming radiation.

El Niño/Southern Oscillation (ENSO)

El Niño is the name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru.

When this warming occurs the usual upwelling of cold, nutrient rich deep ocean water is significantly reduced.

El Niño normally occurs around Christmas and lasts usually for a few weeks to a few months. Sometimes an extremely warm event can develop that lasts for much longer time periods.

The formation of an El NiÑO is linked with the cycling of a Pacific Ocean circulation pattern known as the southern oscillation.

In a normal year, a surface low pressure develops in the region of northern Australia and Indonesia and a high pressure system over the coast of Peru (see Figure below).

This cross-section of the Pacific ocean, along the equator, illustrates the pattern of atmospheric circulation typically found at the equatorial Pacific. Note the position of the thermocline.

As a result, the trade winds over the Pacific Ocean move strongly from east to west.

The easterly flow of the trade winds carries warm surface waters westward, bringing convective storms to Indonesia and coastal Australia.

Along the coast of Peru, cold bottom water wells up to the surface to replace the warm water that is pulled to the west.

In an El NiÑO year, air pressure drops over large areas of the central Pacific and along the coast of South America (see Figure below).

This cross-section of the Pacific ocean, along the equator, illustrates the pattern of atmospheric circulation that causes the formation of the El NiÑO. Note how position of the thermocline has changed from the last figure.

The normal low pressure system is replaced by a weak high in the western Pacific (the southern oscillation).

This change in pressure pattern causes the trade winds to be reduced. This reduction allows the equatorial counter current (which flows west to east) to accumulate warm ocean water along the coastlines of Peru and Ecuador.

This accumulation of warm water causes the thermocline to drop in the eastern part of Pacific Ocean which cuts off the upwelling of cold deep ocean water along the coast of Peru.

The twists and turns in the ongoing dialogue between ocean and atmosphere in the Pacific can have a ripple effect on climatic conditions in far flung regions of the globe.

This worldwide message is conveyed by shifts in tropical rainfall, which affect wind patterns over much of the globe.

El Niño impacts during Northern Hemisphere summer

El Niño impacts during Northern Hemisphere winter

Dense tropical rain clouds distort the air flow aloft (5-10 miles above sea level). The waves in the air flow, in turn, determine the positions of the monsoons, and the storm tracks and belts of strong winds aloft (commonly referred to as jet streams) which separate warm and cold regions at the Earth's surface.

In El Niņo years, when the rain area that is usually centered over Indonesia and the far western Pacific moves eastward into the central Pacific the waves in the flow aloft are affected, causing unseasonable weather over many regions of the globe.

More information and animations are found in the NOAA El Niño Theme page