video Episode 1.1

How we know it’s happening

How do we know the climate is changing? How do we know that change is being caused by human activities? Climate Now video hosts Ozak Esu and James Lawler delve into the science that answers these questions with MIT climate scientist Kerry Emanuel.

Running Time: 7 mins

Date: 05.16.2021

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Kerry Emanuel


Kerry Emanuel


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.

Hosted By:

Ozak Esu
Climate Now Host


Ozak Esu

Climate Now Host

Dr. Ozak Esu is a Chartered Engineer and a STEM Education Ambassador, working within Construction and the Built Environment industry. Inspired by her lived experience of energy poverty growing up in Nigeria, she chose to pursue a career in Electronics and Electrical Engineering, receiving her bachelor’s degree from Loughborough University, UK. She graduated with a first-class and was awarded a scholarship to advance to her PhD. in wind energy. By participating in Climate Now, Ozak is keen to contribute her knowledge of technologies, her experience of the challenges faced, and to be a voice for regions often underrepresented, in the conversations about climate change and a sustainable energy future.

James Lawler
Climate Now Host


James Lawler

Climate Now Host
James Lawler is the founder of Climate Now. James started Climate Now as a way to learn about climate change and our energy system. Climate Now’s mission is to distill and communicate the science of our changing climate, the technologies that could help us avoid a climate crisis, and the economic and policy pathways to achieve net zero emissions globally. James is also the founder of Osmosis Films, a creative studio.

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Video Episode 1.1 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:

  1. The climate is changing in a specific way that is different from the ways in which it’s changed in the past.
  2. 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].” 

0:50 temperature
keeling curve

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?

ice cores

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].

ice layer temp

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].

4:08 sea layers

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].”

5:11 glacial forcing

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.


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

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