source : guardian.co.uk
10 Juni 2011
We often hear strong opinions in the media about climate change. People cast themselves as “believers” or “sceptics” (although I prefer the term “deniers”) — often without a clear understanding of the evidence upon which scientists base their interpretations of how the climate is changing. As a scientist who is using ice core data to study climate change, I would like to explain this fundamental research technique that underlies our understanding of our modern climate. This is part of a larger body of research that provides a detailed context for understanding the climate changes that we are seeing and experiencing today.
Ice sheets are analogous to miles-thick layer cakes of snow that have been compressed under their own weight. Each year, snow falls on the surface of an ice sheet and over time, these layers become buried and are crushed into ice. At the depth where compressed snow transitions fully to ice, the little spaces of air between the grains are sealed off. These wee bubbles have trapped small samples of the air found at this depth. As if sealed in a bottle made of ice, this ancient air still exists. Just like our atmosphere today, these fossil air bubbles contain nitrogen, oxygen, carbon dioxide (CO2), methane (CH4) and other trace gases. To get at these ancient air bubbles, researchers drill a borehole into a glacier or ice sheet to remove a cylindrical “plug” of ice:
Researchers extract these fossil air bubbles to measure the concentrations of each gas, such as the greenhouse gas CO2. By analyzing samples collected throughout the length of a 3,000-metre ice core, scientists have found that CO2 concentrations varied along with temperature during Earthʼs ice ages:
This figure shows historical CO2 (right axis) and reconstructed temperature (as a difference from the mean temperature for the last 100 years on the left axis). Records based on Antarctic ice cores, providing data for the last 800,000 years.
These measurements show that atmospheric CO2 concentrations fluctuated between around 180 and 280 parts per million (ppm) during nearly one million years. This provides an important baseline for understanding our current CO2 level, which is now at 393 ppm (download the NOOA data. [Graph KEY: (red) EPICA DOME C temperature data; (dark blue)Vostok CO2 data; (steel blue) EPICA DOME C temperature data; (pale blue) EPICA DOME C temperature data(cyan) EPICA DOME C temperature data; (arrow) current CO2level].
Ice cores also contain a range of impurities such as microscopic dust particles and minute amounts of chemicals. These impurities, which were present as tiny droplets or particles in the atmosphere, became incorporated into snowflakes or deposited onto the ice through dry deposition, and were then buried in the snow. These dust particles and chemicals provide important clues into past atmospheric chemistry. One example weʼve seen is that atmospheric dust changes dramatically between glacial periods (“ice ages”) and warmer interglacial periods (such as today).
During glacial periods, such as the one that ended around 20,000 years ago, the Earth was drier, colder and windier. During these periods, more dust was blown off continents and deposited on the ice sheets of Greenland and Antarctica. Scientists have even been able to link particularly dusty layers of ice to the activity of glaciers in certain regions (which grind up rocks into powder called ʻglacial flourʼ). Volcanic eruptions are another source of dust that is archived within ice sheets. Eruptions produce sulfate and volcanic ash shards that form distinct layers within the ice. These can be traced to individual eruptions, and used to compare different ice core sites: signatures of particular eruptions can be found in cores from different regions.
Variations in atmospheric sulfur concentrations are an important driver of global climate changes. Volcanic eruptions release large amounts of sulfur dioxide gas (as well as smaller amounts of hydrogen sulfide gas) into the atmosphere. Scientists know that increased levels of sulfur-containing gases cause the planet to cool — for example, large volcanic eruptions such as Tambora in 1815 (which caused the ‘year without a summer’) and Kuwae in 1452 (which was probably twice the size of Tambora’s more famous eruption) caused increased concentrations sulfur dioxide that had abrupt and dramatic effects on global climate. Tracing sulfur dioxide concentrations and atmospheric circulation patterns provides insights into past environmental changes over decadal to millennial timescales.
Perhaps the most important technique associated with ice core research is the measurement of the isotopic composition of water. A full explanation of what isotopes are and how fractionation occurs will take too long to explain here, but there are many good websites that explain the process brief, the ratio of two oxygen isotopes (18O and 16O) in the rain and snow that falls in temperate and polar regions depends on temperature. By measuring changes in the 18O/16O ratio in the ice through time, researchers can infer the temperature at the time when the snow fell. Thus, climate scientists have reconstructed many snapshots of how temperature (and many other parameters) changed year-to-year. By linking all these different lines of evidence together, climate scientists are developing a fairly comprehensive view of how Earthʼs climate varied in the past.
To date, ice cores have been drilled at hundreds of sites in Greenland and Antarctica, and on mountain glaciers located throughout the world. Each location experiences a slightly different climate, so these networks of ice cores give us a more complete understanding of climate dynamics. Evidence from ice cores can be compared to other types of natural archives that record climate history, such as corals, speleothems (layered sediments in caves), tree rings, glacial moraines, deep-sea sediment cores, and lake sediment cores. All of these and others, provide records that help us understand Earthʼs climate history on different time and space scales.
Clearly the topic of ice cores and past climate is too broad to cover in any depth in one article. For more information about specific climate periods of the past (and also as a great overview of the topic), I recommend Dr Richard Alleyʼs book, The Two-Mile Time Machine: Ice Cores, Abrupt climate changge, and Our Future . This book got me fascinated by ice cores when I was an undergraduate.