Minn Lin working on a speleothem sample
Minn Lin receiving her prize from the Dean of Science, Professor Simon Redfern
Lessons in climate change, learned from the past
Written by Wong Minn Lin
Rainfall patterns in response to a changing climate remains one of the hardest climate variables to predict, despite its importance to billions of population’s livelihoods. Particularly in the tropics, where the annual arrival of the monsoon bears great consequence, how can we anticipate the impacts of modern anthropogenic climate change?
At the Asian School of the Environment in NTU, the Isotope Geochemistry group addresses this issue through the perspective of paleoclimate, which is the study of how climate has changed in the past throughout Earth’s history. The Earth’s climate, within its natural variability, has undergone dramatic changes since its early history from being glaciated near the equator at one point, to having forests thrive on Antarctica at another. Even in our planet’s more recent history (still within the past 100,000 years!), glacial states and rapid climate change events were commonplace. At these different points in time, rainfall in the tropics would have behaved differently than they do today and therefore many of these climate states can make analogous situations for our modern and future climate.
But how can we know how the climate behaved long before humanity? Just as there are written archives of human history, there are natural archives of climate histories that document how temperature, sea-level and rainfall have varied in the past. Natural deposits like corals, marine sediments and cave deposits are some examples of climate archives which preserve information of how the Earth’s climate state behaved thousands of years ago. Locked within the chemistry of these natural formations is a data boon from the past. One of the focuses of the Isotope Geochemistry group is reconstructing past rainfall changes using cave deposits known as speleothems, such as stalagmites. Speleothems collected from caves found across the tropics, such as the Asian continent, Maritime continent and tropical South America, can reveal how continental rainfall patterns have shifted in the past in many countries affected by monsoons.
Speleothems are carbonate deposits (CaCO3) that grow slowly over time as rainwater percolates through the ground and into a cave interior. As a result, stalagmites form as the carbonate accretes as layers over hundreds to thousands of years, often forming the beautifully laminated structures shown in figure 1. Just like tree rings, each layer in the rock can represent a point in time. We can then find out the age of these layers through high-precision radiometric dating techniques, the most precise of which uses two elements, Uranium and Thorium, that are naturally found in the carbonate material. Uranium is radioactive and predictably decays to Thorium, so the ratio between these two elements can quite precisely reveal the timeframe in which a speleothem grew. Selected horizons along a sample is analysed using a powerful instrument called a Multi-collector-Inductively-Coupled-Plasma Mass-Spectrometer (MC-ICP-MS) that allows us to get high-precision measurements of Uranium and Thorium for age calculations.
Figure 1. A cross-section of a speleothem sample from Central Brazil. Speleothems grow by the slow accretion of carbonate layers. Red arrows indicate where samples were drilled for Uranium dating. Continuous holes along the axis are drilled for oxygen isotopic measurements.
In addition to a sample’s chronology, we can extract climatic information by relying on a proxy to represent rainfall change. Notice that the chemical formula for carbonate is CaCO3 – it is that oxygen atom that makes a powerful proxy for rainfall strength! Since the cave drip waters are fed from meteorological sources, the oxygen derived from the rainwater (H2O) that makes its way through the ground can eventually be preserved in the carbonate material. Oxygen can come in heavier or lighter varieties, known as isotopes. Because of how the isotopes fractionate during precipitation, periods with weaker rainfall result in more of the heavier isotope,18O, concentrated in the rain. Conversely during times of stronger rainfall the lighter isotope, 16O, is enriched in the rain instead. The ratio between 16O and 18O isotopes, represented the notation of “δ18O”, can therefore act as an indicator for shifts in the amount and/or source of rainfall in the region where the speleothem grew. Again using mass-spectrometers, the oxygen isotopic signal can be extracted along the growth axis of a sample, and when combined with the radiometric dates can produce a graph (figure 2) of how rainfall changed over time! Other types of proxies such as carbon isotopes and trace metal concentrations can also provide a wealth of information on past climatic and environmental conditions above the cave.
Figure 2. Rainfall variability over Central Brazil derived from radiometric dates and oxygen isotopic values (represented with the δ18O -notation) derived from the sample shown in figure 1. The horizontal bars show the dating uncertainties.
Certainly, a single rainfall record from a single cave cannot paint a full picture of past conditions in a whole region. Instead, paleoclimate scientists examine how different paleoclimate records vary comparatively in order other to make sense of regional to global-scale climate dynamics. Such studies can highlight global teleconnections between ocean circulations and atmospheric patterns that are crucial for predicting future climate responses.
Ultimately, an extended climate record can help put present day climate change into context, for example, if any change observed today is within the Earth’s natural variability or not. Most significantly, the combination of model simulations and paleo-data analysis can provide great insight in terms of evaluating the performance of state-of-the-art climate models that we rely on for the monumental task of projecting future climate change. Luckily, the paleoclimate record gives us the advantage of hindsight, learning from a series of events which allow us to test and build our understanding of climate change across time and space. Stay tuned as we navigate the future using the past!