How we know it’s happening
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Kerry Emanuel
MIT
Kerry Emanuel
MIT
Kerry Emanuel is a prominent meteorologist and climate scientist who specializes in moist convection in the atmosphere, and tropical cyclones. His research interests focus on tropical meteorology and climate, with a specialty in hurricane physics. His interests also include cumulus convection, the role of clouds, water vapor, and upper-ocean mixing in regulation of climate, and advanced methods of sampling the atmosphere in aid of numerical weather prediction.
Emanuel received an S.B. degree in Earth and Planetary Sciences and a Ph.D. in Meteorology (1978) both from MIT. After completing his doctorate, he joined the faculty of the Atmospheric Sciences department of the University of California at Los Angeles where he remained for three years, with a brief hiatus filming tornadoes in Oklahoma and Texas. He joined the faculty at MIT in 1981 and is now the co-director of the Lorenz Center and the Cecil & Idea Green Professor of Atmospheric Science.
Professor Emanuel is the author or co-author of over 200 peer-reviewed scientific papers, and three books, including Divine Wind: The History and Science of Hurricanes, published by Oxford University Press, and What We Know about Climate Change, published by the MIT Press.
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In this Episode
So, you might say you believe in climate change – but do you know why you believe in it? Can you state the facts and make the key arguments? Wouldn’t you like to be able to do that, instead of blindly trusting a headline you read on the internet? If so, or if you just need the right words for that pesky uncle at Thanksgiving, or maybe for your next high-stakes political debate in the halls of global power – this video is for you.
So here we go:
To say that human-caused (or, anthropogenic) climate change is real we need to establish two things:
Professor Kerry Emanuel from MIT asks the question thus:
“What is it that we’re saying is different? What we’re saying is different is not the warmth of the planet. It’s been much warmer 55 million years ago than it’s today. It’s not that there are no other reasons climate ever changes. There are plenty of reasons. So can corruptions changes in orbital configurations changes in that sun itself, but we have high confidence that this very high rate of warming by the standards of the geological past is in this case owing to be measured, uh, in contour virtual increase in greenhouse gases, like carbon dioxide and methane in the atmosphere.”
So then, how have we established this very high rate of warming? How do we even know that the planet has been warming in this very rapid way that is abormal by, as Emanuel says, the “standards of geological past.” Because scientists are very, very clever. More specifically:
We have data from a large number of independent sources that makes it clear that the climate is warming at an unprecedented rate. Since the 1960s, we’ve had data from satellites.
Professor Emanuel puts it this way:
“There are so many measurements of temperature by actual thermometers, by satellites, measuring infrared radiation, coming up from the surface, which is a function of its temperature, uh, that you could throw out 80 or 90% of that data just arbitrarily, randomly throw it away and use the rest of it and still get almost exactly the same temperature. Okay. We have a super abundance of data now, as you go back in time, uh, back into the 20th century. And so, uh, it’s still quite good for measuring the kinds of signals are actually detecting until you get back to around the late 19th century.
When the measurements, again, going backward in time are getting sparser and sparser and the error bars, uh, the sampling error bars on, um, what we’re measuring at larger and larger to the point where by the time you’re back around 1850, it doesn’t really mean much. Now having said that, uh, there are other ways we know about the temperature going very far back in the past, that don’t rely on thermometers.”
1. ‘Tiny bubbles in the ice are time capsules, revealing the concentration of CO2, methane and other gases that were present when the ice was formed. Dr. Emanuel: “as the snow compacts into ice, it traps bubbles of air in the ice courts. And we can, uh, take the ice back in the lab, release the air from the bubbles and measure the concentration of gases. And that’s how we know quite accurately, how methane, for example, and carbon dioxide varied, uh, over the lifespan of the ice cores, going back as far as about a million”
2. “Dust from volcanic events is also found in the ice, helping to date the layers using carbon dating of fossils in other places where archeologists have seen the same layers of volcanic dust.”
3. “Heavy isotopes of Oxygen and Hydrogen: Isotopes are forms of elements that have different atomic weights – meaning, the atoms of these elements contain different numbers of neutrons, while still retaining all other chemical properties of those elements. Water – H2O – containing heavier isotopes of Oxygen and Hydrogen, evaporates more slowly and condenses in clouds more quickly than normal water, and so, it turns out there’s a linear relationship between temperature and the ratio of heavier isotopes in precipitation – ice and snow – to lighter ones. So, by determining the concentration of heavy Oxygen and Hydrogen isotopes in ice cores, we can infer the temperature when that precipitation fell, in strikingly high resolution – down to seasonal variations going back nearly 1M years”
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Episode Transcript
So you might say you believe in climate change, but do you know why? Can you state the facts and make the key scientific arguments? Wouldn’t you like to be able to do that instead of blindly trusting a headline you saw on the internet? If so, then this video is for you.
So here we go. To say that anthropogenic or human-caused climate change is real, we need to establish two things:
- The climate is changing in a specific way that is different from the ways in which it’s changed in the past.
- Humans are the cause of this change.
Professor Kerry Emanuel phrases the question this way:
“What we’re saying is different is not the warmth of the planet. It’s been much warmer, 55 million years ago[1]. It’s not that there are no other reasons climate ever changes. There are plenty of reasons: volcanic eruptions, changes in orbital configurations. But we have high confidence that this very high rate of warming, by the standards of the geological past, is owing to the measured, incontrovertible increase in greenhouse gases, like carbon dioxide and methane, in the atmosphere[2,3,4].”
So how do we know that the climate has been warming in this abnormal way? Since the 1960s, we’ve had a massive global network of temperature sensors and weather stations[5]. For 300 years, we have had thermometers, with the first reliable mercury thermometer invented by Dutch scientist, Daniel Gabriel, wait for it, Fahrenheit in the year 1714[6]. And since humans have been taking all these measurements, the climate has warmed by approximately 2℃[7]. But how do we know that this is actually abnormal? How do we know that the climate doesn’t just do this from time to time?
Well, scientists have figured out that by drilling very deep ice cores in Antarctica and New Zealand—literally extremely long cylinders of ice created over hundreds of thousands of years of snowfall and other precipitation—we can determine the temperature that prevailed when those ice layers were formed[8]. One way this is done is by looking at what’s called the oxygen isotope ratio in the ice. Isotopes are forms of elements with different atomic weights, meaning containing more or fewer neutrons, but retaining all other chemical properties of those elements. Water, H2O, comes therefore in different varieties: lighter water (or sort of normal water), and heavier water, containing heavier isotopes of oxygen and hydrogen. Because it’s heavier, it evaporates more slowly and condenses more quickly in clouds.
So the colder it is, the harder it is for water containing heavy oxygen isotopes to evaporate from the ocean and the less of it you will find in polar precipitation, and hence in the ice. If we know the concentration of the heavy oxygen isotopes in the ice layers, we can infer the temperature from when the ice was formed. Amazing, right? And here’s what that data looks like[9,10].
The ups and downs are, you guessed it, the ice ages and the periods of thawing in between. But this isn’t the only way we know what the climate has been in the distant past. Remember that water containing heavier oxygen isotopes gets left behind as lighter water evaporates to form clouds and precipitation. So the more ice there is on the planet, the more lighter water has evaporated from the ocean, the more heavy isotopes of oxygen we expect to see in the remaining seawater. Now, tiny creatures that live in that seawater take in the water and incorporate the oxygen into their shells. So, by looking at the concentration of heavy isotopes of oxygen in the shells of fossilized organisms that we find in sediment layers, we can actually back-out how much heavy oxygen isotope there must’ve been in the seawater when those creatures lived, and from there, we can determine the amount of ice there must have been on the planet[11].
But what’s even more amazing is that the temperatures we get when we look at the ice cores, and the ice volume we expect from the sediment cores actually line up[12]. Colder periods correspond to more ice on the planet and warmer periods to less ice. This is two independent sources of data, two independent signals that are telling us the same thing, exactly when the earth warmed and when it cooled, going back hundreds of thousands, of years.
Kerry Emanuel:
“How do we know that the current warming isn’t part of the glacial cycle? The answer to that question is that the glacial forcing’s headed the wrong way. Records from the Arctic, going back 2000 years, show that the earth was slowly cooling until the dawn of the industrial revolution when it abruptly started to warm[13].”
Today, CO2 concentrations in the atmosphere are above 415 parts per million [14]. At the beginning of the industrial revolution, it was approximately 280 parts per million. To put this into context, 415 parts per million is higher than anything we’ve seen in the last million years, according to data from the ice cores.
So how do we know that humans are the cause of the warming that we’re seeing? In the 1850s scientist Eunice Foote discovered that gases like CO2 are warmed by solar radiation. Scientist John Tyndall proved around the same time that such gases absorb infrared radiation, which is radiated from the earth surface, and so is effectively trapped by those gases, which then creates the global warming effect that we observe[15,16,17].
Kerry Emanuel:
“This is now a question of risk, and to be conservative about risk is almost the opposite as being conservative about signal detection. We know far, far more than we need to now to understand that we’re putting ourselves at risk. It’s like a man about to lead his daughter across a very busy aisle. ‘Prove to me that she’ll be run over, and if you can’t prove it to me, I’m going to let her go!’ It’s not the right approach to risk.”
Check out our full-length podcast with Dr. Emanuel, where we go into further detail on ice core dating, climates of the past, and the future of our energy system, and check out our newsletter. You can subscribe to both via our website, climatenow.com.
Sources
1. Scott, M., R. Lindsey (2020) What’s the hottest Earth’s ever been? NOAA Climate.gov. Published June 18, 2020, accessed March 24, 2021 from https://www.climate.gov/news-features/climate-qa/whats-hottest-earths-ever-been
2. Keeling, C.D., S. C. Piper, R. B. Bacastow, M. Wahlen, T. P. Whorf, M. Heimann, and H. A. Meijer, Exchanges of atmospheric CO2 and 13CO2 with the terrestrial biosphere and oceans from 1978 to 2000. I. Global aspects, SIO Reference Series, No. 01-06, Scripps Institution of Oceanography, San Diego, 88 pages, 2001.
3. Arrhenius, S. (1896) XXXI. On the influence of carbonic acid in the air upon the temperature of the ground, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 41(251), 237-276, DOI: 10.1080/14786449608620846
4. IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group 1 to the 5th Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
5. Warren J.R., D.L. Vance (1981) Remote Automatic Weather Station for Resource and Fire Management Agencies, United States Department of Agriculture Forest Service Technical Report INT-116, Intermountain Forest and Range Experiment Station
6. Bellis, Mary. (2021, February 24). The History of the Thermometer. Accessed March 24 from https://www.thoughtco.com/the-history-of-the-thermometer-1992525
7. NOAA National Centers for Environmental information, Climate at a Glance: Global Time Series, published February 2021, accessed March 11, 2021 from https://www.ncdc.noaa.gov/cag/global/time-series
8. Riebeek, H (2005) Paleoclimatology: The Ice Core Record. Accessed March 24, 2021 from https://earthobservatory.nasa.gov/features/Paleoclimatology_IceCores
9. Lüthi et al. (2008) High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, 453, 379-382. https://doi.org/10.1038/nature06949
10. Jouzel et al. (2007) Orbital and Millennial Antarctic Climate Variability over the Past 800,000 YearsScience, 317, 793-797.
11. University of Michigan Global Change Program, Past Climates on Earth, accessed March 24, 2021 from https://globalchange.umich.edu/globalchange1/current/lectures/kling/paleoclimate/index.html
12. Emanuel, K. (2016) Climate Science and Climate Risk: A Primer. Accessed March 25, 2021 from: https://emanuel.mit.edu/
13. Kaufman, D et al. (2009) Recent Warming Reverses Long-Term Arctic Cooling. SCIENCE (325) 1236-1239, DOI:10.1126/science.117983
14. Dr. Pieter Tans, NOAA/GML (gml.noaa.gov/ccgg/trends/) and Dr. Ralph Keeling, Scripps Institution of Oceanography (scrippsco2.ucsd.edu/)
15. R. P. Sorenson (2011) Eunice Foote’s pioneering work on CO2 and climate warming, Search and Discovery article #70092, accessed March 24, 2021 from http://www.searchanddiscovery.com/documents/2011/70092sorenson/ndx_sorenson.pdf
16. Wells, David A., ed. (1857) Annual of scientific discovery: or, year-book of facts in science and art, for 1857, exhibiting the most important discoveries and improvements in mechanics, useful arts, natural philosophy, chemistry, astronomy, meteorology, zoology, botany, mineralogy, geology, geography, antiquities, etc., together with a list of recent scientific publications; a classified list of patents; obituaries of eminent scientific men; notes on the progress of science during the year 1856, etc.: Gould and Lincoln, Boston, 159-160, accessed April 2, 2021 from https://archive.org/details/annualscientifi01nichgoog/page/n169/mode/2up?q=Foote
17. Foote, E. (1856) Circumstances affecting the Heat of the Sun’s Rays. The American Journal of Science and Arts, XXII, 382-383, accessed April 2, 2021 from https://static1.squarespace.com/static/5a2614102278e77e59a04f26/t/5aa1c3cf419202b500c3b388/1520550865302/foote_circumstances-affecting-heat-suns-rays_1856.pdf