Vikings from Norway and Iceland were the first to discover Greenland about 1000 years ago. Disko Island, were we now are, was one of the favorite hunting grounds of the settlements of Nuuk. Eric the Red who was the first to discover these lands, was a true salesman. To attract his fellow citizens living on Iceland, he called the island Greenland. Perhaps Eric was true to his word, and Greenland was greener and warmer about 1000 years ago? However, how do we know this, when this is a period where we don’t have any instrumental records? This will be the topic of this week’s lecture.
Direct meteorological measurements of properties such as temperature, wind and ocean currents only exist for the past two centuries. To reconstruct climate for periods older than this, we rely on climate proxies. These proxies are known to represent physical properties of the climate system and are preserved in a variety of different archives such as ocean sediments, lake sediments, and ice cores.
On the continents, rainfall and runoff slowly erode the underlying bedrock. The sediments produced are efficiently transported by streams and rivers to lakes, interior seas and eventually to the ocean. When the sediments pass through areas of quiet water, they are deposited as successive layers. If undisturbed, these packages of layered sediments represent a unique archive, which can be extracted and sampled to reconstruct past climate. Many lakes at high latitudes, such as here in Norway are fed by glaciers. These efficiently carve the landscape and leave behind moraines, which are a jumble of unsorted boulders at the margin of the ice, as well as large amounts of silt.
This fine silt is transported downstream with the meltwater in rivers and deposited as layers of sediments in glacier fed lakes. Drilling into these lake sediments one can find pollen from ancient vegetation and by analysing the physical properties of the sediments, one can construct a picture of climate since the last ice age, and sometimes even further into the past. As compared to land, the ocean is not effected by erosion due to ice and running water. Therefore, the sediments deposited in the ocean are better preserved, and can be many million years of age. This leaves a fantastic archive of past climate, which scientists can retrieve by drilling cores.
Today we have many hundred such sediment cores, from different locations throughout the ocean basins. Most of these cores are less than 50 meters long, and as the ocean sediments are relatively soft, the cores can be extracted without the need for drilling. Depending on the sedimentation rate, the cores can cover a few thousand, to several million years of age.
Now you might ask yourself; how can these sediment archives be used to reconstruct climate? It turns out that plankton which are abundant in surface waters and at the bottom of the ocean form shells of calcite. Once the plankton die, and fall to the sea floor, the calcite shells are preserved in the sediments. Analyzing the species of plankton shells found in the sediment cores as well as isotopic analysis of the oxygen molecules in the calcite, can give an approximate temperature of the surrounding waters when the plankton lived. In this way we can reconstruct ocean temperatures at different depths and different locations in the ocean back in time.
Besides sediment archives, the best climate reconstructions come from ice cores drilled into the two major remaining ice caps covering Greenland and Antarctica. As in the ocean where there is a continuous deposition of sediments, there is a continuous deposition of snow on the surface of these ice caps. This snow is preserved and turns into ice as it is compressed by new layers of snow. When drilling to the bottom of the 3km thick Greenland ice cap, one finds ice that is just beyond 125,000 years of age. On Antarctica, where the accumulation of new snow is very slow, the deepest ice cores, which are about 3.2km long, are as old as 1 million years.
Based on the oxygen-isotope thermometer from deep sea sediments, we now know that climate has been much warmer in the past. 65 million years ago, there were no ice caps on the continents, and no sea ice in the ocean. Instead, there were tropical forests and crocodiles north of the Arctic circle. Following this warm period, climate gradually cooled, with the first ice caps on Antarctica appearing about 35 million years ago. This was followed by frequent glaciations of North America and Eurasia starting about 3 million years ago. During the past 3 million years, repeated glaciations have dominated the climate. However, how do we know that continents were once covered by several kilometers of ice? The first clues came from studying erratic boulders.
These are large boulders of a distinct rock type not found locally, indicating that they have been carried from the source and deposited many kilometers away from the glaciers. One of the pioneers in this field was Jens Esmark, a Danish-Norwegian professor of mineralogy who in 1824, based on surveying erratics and moraines, postulated that glaciers once covered much of Norway. This was later confirmed by the work of the famous Swiss biologist Louis Agassiz, who found that there had been ice caps covering much of the European continent. By studying the sediment archives and ice cores, scientists have learnt that the glaciations of the past 3 million years have a regular periodicity.
Early on, the glacial cycles were relatively small and repeated every 41 thousand years. During the past 1 million years, the glacial cycles gradually increased in amplitude, and the duration of each glacial period has become closer to 100 thousand years.
Not only do we know how temperature and ice varied in the past. With the help of ice cores we can also measure the atmospheric content of greenhouse gases, such as CO2. This is because ancient air is trapped and preserved as tiny bubbles in the ice. By analyzing the longest ice cores from Antarctica, we find that atmospheric CO2 has varied together with atmospheric temperature throughout the past 800,000 years. During the past warm periods, such as the interglacials, atmospheric CO2 was relatively high at 280ppm. And during past glacial periods, atmospheric CO2 was low, with a value of approximately 180ppm. Today, we live in an interglacial, which has lasted about 10,000 years.
However, in the past 150 years since the start of the industrial revolution, levels of CO2 in the atmosphere have increased well above 280ppm.
As you have learnt in the previous lectures, the main driver of past changes in climate, including the glacial cycles, is the Sun. The long term cyclic changes in solar radiation is caused by changes to the tilt, as well as the slow wobble of the Earth’s spin axis with respect to the fixed stars. As we circle the Sun, the amount of radiation received on the surface of the Earth changes. If you pick at specific latitude and season, for example, summer solstice at 65 degrees north which is shown here, the changes in radiation are significant and are believed to have had a large impact on the growth and decay of land based ice sheets.
This is demonstrated by the climate record showing repeated glacial cycles over this period. Now, the cyclic variations in the Earth’s orbit are too slow to explain changes in climate over periods shorter than a few millennia. However, the proxy records show significant regional changes in climate during the past few thousand years. This is partly due to changes in volcanism, and the frequency of Sunspots. As an example, the relatively warm period about one thousand years ago, when Erik the Red settled Greenland, is probably a combination of low volcanic activity and high solar activity.
As we have learnt in this lecture, climate can be reconstructed from marine sediments as well as ice cores. Based on these archives, we know that climate has been cooling over the past 65 million years. Starting with the warm greenhouse climates at the time when dinosaurs roamed the planet, until the glaciation of Antarctica about 35 million years before present. In the Northern Hemisphere, ice only started to grow about 3 million years ago. Today, the only ice that remains is that of Antarctica and Greenland. However, the long term future of these ice sheets are uncertain, and depend on the combined effects of natural as well as human induced radiative forcing.