How has the climate changed?

Past climate has changed enormously on different time scales.

Scientists use ice core samples to reconstruct climate records.

Scientists have used special devices to measure and record weather and climate since the 1850s. NASA's Global Precipitation Measurement (GPM) Core Observatory satellite was designed to provide rain and snow observations worldwide.

Earth's climate has changed dramatically many times since the planet formed about 4 billion years ago. These changes were triggered by the changing configuration of continents and oceans, changes in the intensity of the sun, fluctuations in the Earth's orbit and volcanic eruptions.

Natural fluctuations in the concentration of greenhouse gases in the atmosphere, the evolution of life and meteorite impacts have also caused climate change in the past. For example, millions of years ago, the global average temperature was a few degrees higher than it is today, and warm, tropical waters stretched much farther from the equator, resulting in very different ocean and atmospheric circulation patterns than today.

Over the past million years, the globally averaged surface temperature of the Earth has risen and fallen by about 5 °C in glacial cycles about every 100,000 years. During the coldest period of the last ice age, about 20,000 years ago, sea levels were at least 120 meters lower than they are today because more water was trapped in polar ice sheets on land. The past 8,000 years, which make up most of recorded human history, have been relatively stable at the warmer end of this temperature range. This stability made possible agriculture, permanent settlements, and population growth.

Most past global temperature changes have occurred slowly, over tens of thousands or millions of years. However, there is also evidence that some abrupt changes have occurred, at least at the regional level. For example, during the last Ice Age, temperatures in the North Atlantic region changed by 5 °C or more in just a few decades, likely due to sudden collapses of the Northern Hemisphere ice sheets or changes in ocean currents.

Past temperature changes are consistent with changes in CO2 levels on different time scales. These graphs show changes in long-term average temperature and average atmospheric CO2 concentration (parts per million) over the last (a) 800,000 years, (b) 2,000 years, and (c) 160 years. Temperature changes in (a) refer to Antarctica, while (b) and (c) are global averages.

Treeing

Tree rings provide a source of climate change data spanning hundreds of years.

Previous records show that global climate is sensitive to small but persistent influences.

Glacial cycles were triggered by small variations in the Earth's rotation and in its orbit around the Sun. These altered the seasonal and latitudinal distribution of solar energy reaching the Earth's surface. Measurements from climate archives such as ice cores show that changing temperatures changed other climate factors such as the concentration of carbon dioxide (CO2) in the atmosphere, amplifying the initial disturbances. During warm periods, the major greenhouse gases CO2 and methane were released into the atmosphere and retreating ice sheets reflected less sunlight into space. These observations confirm that the climate system is sensitive to small perturbations that can be amplified by amplifying feedback processes. Likewise, today's climate system is sensitive to disturbances caused by human influences.

How do we recognize climate change?

Identifying global temperature changes requires frequent observations from many locations around the world. Thermometers, rain gauges, and other simple instruments have been used to measure climate variables since the mid-19th century. Over time, the quality, variety, and quantity of observations has improved. Since the 1970s, sophisticated sensors on Earth-orbiting satellites have enabled nearly global recording of many climate variables. By carefully analyzing the data collected with these techniques (while carefully accounting for changes in instrument types, observation practices, instrument locations, and urban areas), it has been possible to map the distribution of temperature and other climate changes since the late 19th century.

To study climate changes that occurred before direct measurements were made, scientists use indirect evidence from other sources that record a climate signal. These include climate signals encoded in the composition of ice cores, corals, ocean and lake sediments, and tree rings. All of these records made sequentially over time as an organism grows or sediment accumulates. Ice cores from polar ice sheets, composed of snow deposited over tens to hundreds of thousands of years, provide records of both CO2 and past temperatures. As the snow turns to ice, it traps air in sealed bubbles that provide a sample of past atmospheric composition, while the ratio of stable isotopes of oxygen or hydrogen in the water molecule depends on the temperature at the time the snow fell. Recent historical changes can be identified by analyzing written and pictorial records, such as B. Changes in glacier extent.

Has global warming stopped recently?

By most estimates, the rate of average surface warming has slowed since 2001, despite continued increases in greenhouse gases. This slowdown is consistent with known climate variability. Indeed, decades have been observed over the past century when little or no temperature trend has overlaid the long-term warming trend.

Two main factors have contributed to the recent period of surface warming slowdown. First, subtraction variability in the ocean-atmosphere system has redistributed heat in the ocean, particularly in the eastern and central Pacific. This has resulted in intense warming and cooling of the surface waters and lower atmosphere in this region. Second, several temporary global cooling influences have come into play, including unusually weak solar activity, increased aerosol production, and volcanic activity.

None of these influences are expected to last in the long term. In addition, despite the slowdown in surface warming, there has been a continued increase in heat extremes and ocean heat content, as well as rising sea levels, shrinking Arctic sea ice, and continued melting of ice sheets and glaciers. Some models predict renewed warming will occur rapidly when the current slowdown ends.

Scientists use ice core samples to reconstruct climate records

Scientists use ice core samples to reconstruct climate records spanning hundreds of thousands of years.

Average global temperatures have increased over the last century.

Climate and sea levels were relatively stable, albeit with some fluctuations, over thousands of years of recorded human history up to the 19th century. However, between 1850 and 2012, the global average surface air temperature increased by about 0.8 °C. The rate of warming increased in the mid-1970s, and since 1850 each of the last three decades has been warmer than all previous decades, with the last decade being the warmest of these. Satellite observations and direct measurements also show warming of the lower atmosphere over the last three decades. In contrast, the atmosphere above about 15 km (the stratosphere) has cooled during this time.

The temperature of the oceans has also risen. More than 90% of the heat accumulated in the climate system between 1971 and 2010 was stored in the oceans. Most of the ocean warming occurred near the surface, with the top 75 m of the ocean warming at an average rate of 0.11 °C per decade between 1971 and 2010.

Changes are evident in many parts of the climate system

Changes associated with increases in global temperature have been observed in many other components of the climate system.

Mountain glaciers have been shrinking since about 1850, contributing to global sea level rise. Melting accelerated significantly in the 1990s.

Both the Greenland and West Antarctic ice sheets have lost ice since 1990. This is due to increased ice entry into the ocean and also increased surface melt in Greenland. The loss rate from Greenland appears to be increasing.

The area of the Arctic Ocean covered with sea ice has decreased significantly since 1987 all year round, especially in summer. The thickness of the ice has also decreased by more than 30% in the last 30 years.

In the Southern Ocean, there are strong regional differences in the change in areas covered by sea ice, but a small increase in total cover, driven by shifts in winds and ocean currents in a warming Southern Ocean. The strengthening of circumpolar winds around Antarctica has also been linked in part to depletion of the ozone layer.

The sea surface in rainy parts of the world is becoming less salty, which coincides with the dilution of fresh water from increased precipitation.

Some ocean currents have changed in response to changes in surface winds, sea temperature, and ocean salinity. The changes include a southward shift in the Antarctic Circumpolar Current and an increasing southward penetration of the East Australian Current.

More and more plants and animals on land and in the oceans are experiencing shifts in their distribution and life cycles that coincide with observed temperature changes.

There are regional differences in climate change, including within Australia.

In the last 100 years, temperatures have risen almost everywhere in the world; The rate of increase was greatest in continental interiors. Average surface temperatures over the Australian continent and surrounding oceans have increased by nearly 1 °C since the early 20th century. Seven of the ten warmest years on record in Australia have occurred since 2002. However, there are differences between Australia, with some regions having warmed faster and others showing relatively little warming.

Since the mid-1990s there has been a marked increase in wet season precipitation over north-west Australia, a decreasing trend in south-west Australia and a 15% fall in late autumn and early winter precipitation in the south-east

Temperatures have been rising over Australia and the surrounding ocean since the early 20th century, although there are regional variations. The graph at left shows deviations from the 1961–1990 average sea surface temperature and overland temperatures in the Australian region; The map on the right shows the distribution of annual mean temperature change across Australia since 1910.