CSIRO Atmospheric Research

Climate variability and climate change — the last 120 million years
CSIRO Atmospheric Research Greenhouse Information Paper

In the long-term past, the planet has been both warmer and cooler than today. Climate fluctuations arise because of changes to the Earth’s orbit, to solar radiation, to the positions of continents, and to concentrations of atmospheric greenhouse gases.

More recently, the 20th century was the warmest of the past 1000 years. The 1990s were the warmest decade of the millennium in the northern hemisphere, and 1998 was the warmest year.

Greenhouse gas increases were the main factor contributing to global warming in the late 20th century.

This information paper examines how global climate has changed throughout history and describes the factors likely to be causing the changes experienced during the past 100 years.

Climate throughout history

The past 120 million years

The Cretaceous Period (120-65 million years ago) was 5-7oC warmer than present and carbon dioxide concentrations were much higher than today. Cooling then occurred during the Tertiary Period to the Quaternary Period (2.5 million years ago). During this time, climate was influenced by the location and configuration of continents, the presence or absence of ice sheets, changes to the Earth’s orbit that affected radiation received from the Sun, and high greenhouse gas concentrations.

Geological evidence shows the Earth’s climate is very sensitive to changes in concentrations of greenhouse gases. Models cannot accurately simulate climate of the past 120 million years without incorporating the heat trapping capacity of these gases.

The past one million years

Most of the past one million years was characterised by a series of changes from ice ages (glacial periods) to interglacial periods about every 100,000 years and other variations with periods of about 40,000 and 20,000 years. These coincide with variations in the Earth’s orbit — the so-called "Milankovitch" cycles — that affect the amount and the distribution of solar radiation reaching the Earth.

However, changes to solar radiation alone cannot explain the size, rate and distribution of past temperature changes revealed by evidence from ice cores, sea and lake sediments and continental records.

During the past four transitions from glacial to interglacial periods (Figure 1), changes to the Earth’s orbit initiated warming, which was amplified first by increased greenhouse gas levels and then by reduced reflectance as northern hemisphere ice sheets melted several thousand years later. Concentrations of carbon dioxide and methane increased almost in phase with the glacial-interglacial warmings. Carbon dioxide concentrations lagged the coolings during glacial onset. However, the exact phasing of temperature and carbon dioxide changes is not the main issue. The important finding is that greenhouse gas variations caused up to half of the amplitude of the temperature changes over the glacial-interglacial periods.

Surges of ice and melt water from large ice sheets also had significant and abrupt effects on climate during or immediately following glacial periods, mainly by altering ocean circulation patterns.

Figure 1: Changes to solar radiation levels, surface temperature, methane and carbon dioxide concentrations deduced from ice cores extracted from Vostok in Antarctica. There have been four ice ages and intervening warmer periods during the past 400,000 years. (Source: Nature)

The past 1000 years

Figure 2. Temperatures of the northern hemisphere for the past 1000 years. (Mann, M.E., R.S. Bradley and M.K. Hughes (1999) Northern hemisphere temperatures during the past millennium: inferences, uncertainties and limitations. Geophysical Research Letters, 26, 759-762.)

Evidence of climate variability is more abundant for the past millennium and the factors that affected climate are better understood. Figure 2 presents northern hemisphere temperatures based on a range of measurements. Twentieth century warming runs counter to a slight millennial-scale cooling trend expected from changes to the Earth’s orbit.

The past 150 years

Figure 3 shows direct observations of global temperatures since the 1850s.

Figure 3. Globally-averaged surface temperatures from the instrumental record. (Source: Climatic Research Unit and the UK Met. Office Hadley Centre)

The main factors influencing climate since the mid-19th century include aerosol — tiny particles suspended in the air — from volcanoes and human sources, as well as from solar fluctuations, stratospheric ozone depletion and greenhouse gas increases. El Niño – Southern Oscillation events also affect climate. However, the changes in surface temperature since the mid-19th century can only be accurately simulated by models that include greenhouse gases and aerosols resulting from human activity.

Greenhouse gas increases are considered to be the main factor contributing to the warming observed in the late 20th century. Of the greenhouse gases increasing in concentration due to human activity, carbon dioxide is likely to have caused the most warming, followed by methane, tropospheric (lower atmosphere) ozone, halocarbons and nitrous oxide.

Aerosol arising from burning fossil fuels have a cooling effect that partly offsets warming due to carbon dioxide.

November 2000

Site updated 1st October 2003

Modified: April 3, 2008

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