Sometimes to move forward you have to go back – sometimes as far as hundreds of thousands or millions of years. This is the case for scientists looking to reconstruct the past in order to better understand climatic changes happening today and what may be expected in the future.
What researchers are finding is that while the Earth’s climate has naturally ebbed and flowed between warm and cold, the rate of change observed today is a new phenomenon.
“We are changing climate on scales of tens or hundreds of years, [while] it took natural variations millions of years to get where we are today,” said Colin Jones, a Ph.D. candidate at the University of Rhode Island’s Graduate School of Oceanography (GSO), at a recent Bay Informed Discussion Series talk. He described the science behind paleoceanography and ongoing research to better understand the history of the oceans and their role in climate.
Today, carbon dioxide levels have surpassed 400 parts per million (ppm) – a level that has not been observed in human history and shows no signs of abating as continued emissions into the atmosphere outpace what can be naturally absorbed by ocean and land-based processes.
How do scientists know this?
Researchers have been collecting data on temperature since the mid-1800s and on carbon dioxide since the 1960s, studying the links between mass carbon emissions from fossil fuels and its influence on land and sea-surface temperature. But to go further back in time, scientists needed to do some digging.
“We can directly measure carbon dioxide concentration in a bubble of air trapped in ice cores going back 800,000 years,” said Casey Hearn, a Ph.D. candidate at GSO who co-presented with Jones. “We don’t have anywhere else in the 800,000-year record where we’ve been above 400 ppm in carbon dioxide.”
These air bubbles can provide scientists with a direct measurement of atmospheric gas concentrations (carbon, nitrogen, and oxygen) from a certain time, but what about temperature? In order for scientists to gather more information on past climate conditions they can’t directly measure, and to go further back in time, they rely on proxies or information they can use to infer a change in something closely related.
For example, nitrogen, carbon, and oxygen all have varying isotopic signatures, which means there are various forms of the same molecule that have slightly different atomic masses because they contain different numbers of neutrons. The differences in atomic masses among these isotopes affect the way each molecule behaves within its environment, giving scientists an insight into the physical and biological processes of a certain time period and associated climate conditions. These isotopes can be found in ice and sediment cores, as well as fossilized marine organisms, which can indicate past climate conditions, including temperature.
The damage has been done, changes have been made, but the degree to which things will change further really has to do with where we take it from here.
Oxygen-16 (O-16), which accounts for 99 percent of the oxygen found in air and water, has 8 neutrons and 8 protons. It is lighter and can evaporate more easily than oxygen-18 (O-18), which is heavier, with two extra neutrons, and precipitates out into rain more easily. When the climate is warmer, seawater contains more O-16. When it is colder, scientists will find more O-18, as it either doesn’t evaporate or is rained out at the lower latitudes before it can become snow in the colder regions.
“When you have ice on land, the snow is enriched in O-16 because all the heavy stuff [O-18] has fallen out and O-16 is getting stuck in the ice, and the ratio of it changes in the ocean,” Hearn said, noting that this kind of record is most readily found in fossilized marine organisms that use oxygen in the water to make their calcite-based shells. The oxygen ratios found in those shells indicate the ratio present in the seawater at a certain time, reflecting associated temperatures.
Nitrogen isotopes respond to biological processes and add complementary information to that provided by oxygen isotopes to help scientists better understand past climates.
“If you have a garden, or grow anything, you know you need to fertilize it. And just like the plants on land that need nitrogen to grow, plants in the ocean need that as well,” said Jones, explaining how nitrogen isotopes stored in microscopic phytoplankton buried deep in ocean sediment indicate productivity and nutrient levels, allowing scientists to look at marine biological activity in the past and associated climate conditions. “It can help inform us on what drives climate change and what are factors in big shifts.”
But because proxies are indirect measurements, it’s easy to question their validity, which is why researchers have to be careful in interpreting how the various processes work.
“We test proxies by how they [behave] now and how this reflects physical, chemical, and biological processes in the past,” Jones explained, adding that by using multiple proxies together, such as oxygen and nitrogen, scientists can better understand if past climate conditions were fundamentally different than they are today.
Researchers like Hearn and Jones who study past climates from Antarctica to here in Narragansett Bay are trying to determine how the Earth’s systems are responding to rapid changes and how they will continue to change on a narrower timescale.
“There are some interesting examples in our records of relatively sharp changes which give us an idea of how the earth system responds when there is sudden jump–when something really dramatic happens–what changes you can expect to biology, to the chemistry of the oceans,” said Hearn, noting that even those “sudden” changes happened much more slowly than today. “We are running a crazy experiment with our own planet. We are changing the concentration of gases in our atmosphere in such a profound way that the natural system simply hasn’t done, at least in the last 800,000 years.”
“The further back we can get in time the more examples we’ll have to understand those processes better,” he added. “The severity of the changes that we’ll see is really up to us. The damage has been done, changes have been made, but the degree to which things will change further really has to do with where we take it from here. Are we going to continue burning fossil fuels until they’re all gone? Are we going to make the switch to renewable sources of energy? The rate at which we do these things is really important, so tipping point or not, we have a tremendous amount of control over what happens in the long run.”
Watch full presentations
[info]The Bay Informed Discussion Series is supported by Rhode Island Sea Grant in partnership with GSO. This series is held every third Thursday of the month at 7 p.m. at URI’s Graduate School of Oceanography’s Bay campus in Narragansett. These events are designed for the community to get involved and learn more about research at GSO.
The next event will be on November 16: Warming Polar Regions
Note: There is no December event.
– Meredith Haas | Rhode Island Sea Grant Research Communications Specialist