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| Earth on Dec 20, 2012

Temperature records from nature reaffirm climate warming

In a large compilation, scientists used used 173 independent datasets – from natural sources such as ocean sediments – to show warming over the past century.

A team of scientists has reaffirmed that Earth’s climate has been warming for the past century, using an analysis of temperature records other than those from instruments. These scientists – including researchers from NOAA’s National Climatic Data Center (NCDC), University of South Carolina, University of Colorado, and University of Bern in Switzerland – gathered temperature records from nature to show warming on Earth from at least 1880 to 1995. They say this study resolves some of the uncertainty associated with thermometer records, which can be affected by land use changes, shifts in station locations, variations in instrumentation and more. They published their research online this week in Geophysical Research Letters.

Green line represents the research through paleo records. Black line indicates temperature readings that were recorded with thermometers since 1880. Image via NOAA.

When it comes to researching climate and temperature trends, scientists have always used more than instrument readings to investigate Earth’s temperature from centuries past. For example, they also use what they call paleo-proxy reports – found in cave stalagmites, tree rings, layers accumulating in ice caps, ocean and lake sediments, and corals – which not only provide temperature recordings from across the world, but also provide a comparison with what thermometers have recorded. In this large compilation, scientists used used 173 independent proxy datasets to draw a temperature record from 1730 to 1995. The results show that warming has been occurring throughout the past century.

Here are a few examples of what the scientists analyzed to create this study:

A chemical analysis of corals reveals conditions in the ocean when each layer of coral formed. Image via Richard Ling and Wikimedia Commons.

Coral: Coral skeletons are made of calcium carbonate, which is a mineral extracted from sea water. Scientists locate this carbonate in coral in order to be able to measure the isotopes of oxygen within it. These chemicals reflect conditions in the ocean when each layer of coral formed. They can indicate how temperatures changed throughout the period the coral was alive. Look here for more about how corals indicate past climate.

Ice Cores: Ice cores used in this study came from locations both high on mountaintops and deep within polar ice caps. The ice cores withdraw from these places are essentially snowfall accumulation, built up over centuries. Scientists drill into the ice, and collect the cores, which contain isotopes of oxygen, dust and air bubbles that can give us a decent approximation of temperatures during a certain time period. More details on the ice core record here.

Ice core in which annual layers are clearly visible. The layers result from differences in the size of snow crystals deposited in winter versus summer and resulting variations in the abundance and size of air bubbles trapped in the ice. More about this image at Wikimedia Commons.

Sea floor core sample labelled to identify the exact spot on the sea floor where the sample was taken. Slight variations in location can make a difference in the chemical and biological composition of the sediment sample. Image via Wikipedia

Ocean and lake sediments: Scientists also drill into sediments located on the ocean floor. Approximately six to 11 billion metric tons of sediments accumulate in oceans and lake basins each year. The materials in these sediments consist of things that were produced in the ocean/lakes and also materials that were washed in from nearby land. The chemicals and tiny fossils that blended with the sediments can be used to determine temperatures and perhaps weather conditions over a long period of time. To learn how scientists study ocean sediments, click here.

Paleoclimate records such as these are affected by multiple environmental influences, not just warming, and the scientists minimized non-temperature influences by averaging together many records.

The overall results indicate that this study confirms the instrument records over the past century. Both natural records and instruments show a warm-up in the 1940s, then a dramatic increase in the rate of change in warming temperatures from 1980 through 1995. The instrument record shows a rapid rate of warming after 1995 as well, but this particular study did not extend to the present.

This study reaffirms what we already know. Earth is getting warmer. There is nothing in this study that points to anthropogenic warming or human-caused warming, by the way. However, a majority of climate scientists do accept that humans are a large contributing factor to the warming temperatures experienced globally in recent decades.

The video below is from NOAA and explains more.

Bottom line: A team of researchers from NOAA’s National Climatic Data Center (NCDC), University of South Carolina, University of Colorado and University of Bern in Switzerland looked at paleo-proxy records such as ice cores, coral skeletons, and ocean and lake sediments to figure out how temperature has changed over the past several centuries. They used 173 independent proxy datasets to draw a record from 1730 to 1995. The results from this study reaffirm that warming has occurred throughout this past century, with readings indicating an increased rate of warming from 1980 to 1995.

Read more from NOAA: Independent evidence confirms global warming in instrument record