20.8: Climate Proxies and the Climate Record
Climate Proxies and the Climate Record
For many of us, the most obvious method to study climate would be to record it directly. For example, we could use a thermometer to directly measure air temperature; however, thermometers only provide a localized measurement, and more importantly, they are a relatively recent invention. How do we measure the temperature of the Cretaceous, Permian or Devonian atmospheres or oceans? Direct observations do not give us the long-term trends we need to establish climate change patterns; instead, we must look at a natural recorder of climate called a climate proxy Links to an external site.. As climate changes, it affects the deposition of sedimentary rock, rock chemistry, and fossil organism, all of which can be studied by geoscientists to reconstruct ancient climate patterns in a field called paleoclimatology Links to an external site..
An individual climate proxy may not provide a clear signal of global climate for a couple of reasons: 1) proxies show a history of the area in which they were formed, not of an entire region, 2) an individual proxy, which may have a long or a short record, can record the short-term variability of weather events, and 3) most climate proxies are influenced by multiple factors. For instance, the thickness of tree rings, dendrochronology Links to an external site., is a wonderful proxy for temperature. Trees grow more in warmer years, producing thicker rings, and less in colder years, producing thinner rings; however, a tree could also grow slowly because of a drought or because of an infestation of pests even if it was a warm year (Figure 20.10).
Figure 20.10: The color and width of tree rings can provide insight into past climate conditions. (Public Domain; NASA Links to an external site.)
If all individual proxies show local patterns, with some degree of weather-related noise, and possibly influenced by other factors, how can we reconstruct long-term global temperature records? The answer lies in increasing the size of the dataset. If temperature is the most important variable influencing the proxies, and we combine hundreds to thousands of individual proxy records, an overarching pattern emerges. Again, an individual proxy record may be contrary to the overall trend, but that is to be expected since a local region can have a cold winter during an overall hot year for the planet. To illustrate this, consider the following: say we want to reconstruct overall economic patterns over the past few hundred years in the United States of America. We could examine lots of proxies for economic growth, like employment, the stock market, individual wealth, or rates of home ownership. If we only looked at one of these proxies, we likely would not get a clear picture of change. Also, if we only looked at Los Angeles, California, for example, we would be unlikely to see a trend that mimics the entire country. Again, the more data we have, whether for climate or any other complex system, the clearer the signal becomes over the local and random noise.
One of the most commonly used climate proxies is the measurement of oxygen isotopes Links to an external site.. As you may remember, isotopes Links to an external site. are atoms of the same element that differ in their masses because of differences in the number of neutrons in the nucleus of the atom. Multiple isotopes of oxygen are stable, meaning they do not radioactively decay over time. Oxygen has two stable isotopes that occur in a constant ratio on Earth; however, certain minerals, like calcite or ice, prefer one isotope over the other within their crystal structure. This preference results in a ratio of oxygen isotopes that is different from the ratio found in other materials; this difference is called fractionation Links to an external site.. The amount of fractionation in oxygen isotopes is temperature dependent, such that the mineral calcite has a different ratio of oxygen isotopes if it was formed in near-freezing versus warm water. Oxygen isotopes provide climate records from many different sources, including coral, clams and other mollusks, the skeletons of single-celled organisms (foraminifera and coccolithophores), and ice cores to name a few (Figure 20.11). Ice cores contain a wealth of climate data in addition to temperature data from oxygen isotopes; they also include air bubbles that record the levels of greenhouse gases, concentrations of windblown aerosols, and ash from volcanic eruptions.
Figure 20.11: Water vapor gradually loses 18O as it travels from the equator to the poles. Because water molecules with heavy 18O isotopes in them condense more easily than normal water molecules, air becomes progressively depleted in 18O as it travels to high latitudes and becomes colder and drier. In turn, the snow that forms most glacial ice is also depleted in 18O. As glacial ice melts, it returns 16O-rich fresh water to the ocean. Therefore, oxygen isotopes preserved in ocean sediments provide evidence for past ice ages and records of salinity. (Public Domain; Robert Simmon/NASA/GSFC Links to an external site.; modified by Chloe Branciforte)
Other proxies include the extent of glacial sediment, sea level curves, pollen (palynology), and fossils. For instance, climatologists have used several features within fossil plants to reconstruct climate, largely because these organisms are sensitive to climate. These proxies include the thickness of tree rings, the shape of the leaves, and the density of pores on leaf surfaces.
By combining hundreds to thousands of individual climate records, climate scientists begin to gain insight into overall climate trends. The Intergovernmental Panel on Climate Change (IPCC) Links to an external site. and the National Oceanic and Atmospheric Administration Links to an external site. (NOAA) regularly compile multiple types of proxy records from across the world to reconstruct climate patterns. The accuracy of climate records very much depends on the time frame being considered, with more certainty in the patterns of the recent past (Cenozoic) and less the further back in geologic time (Figure 20.12).
Figure 20.12: This figure shows the long-term evolution of oxygen isotope ratios during the Phanerozoic eon as measured in fossils. (CC-BY-SA 3.0; Global Warming Art Links to an external site.)
There is no scientific debate surrounding the interpretation of individual proxies and the resulting climate records; however, there appear to be many-sided discussions that largely stem from the economic and political aspects of climate change. It is important to remember to consider how scientific data is presented to the public and how we make conclusions based on presented data. When presented with new data or information it is important to consider 1) the source of the data, 2) how the data was collected, 3) how the data is presented, and, most importantly, 4) what reasonable conclusions can be drawn from the data independent of opinions. Lastly, as humans who share a planet with many, we must be willing and open to a change in perspective, particularly when faced with new data and information.