Citizens ask the expert in climate physics

24 11 2020

In the first of two consecutive interviews with climate-change experts, authors, editors and readers of the Spanish magazine Quercus have a chat with Ken Caldeira, a global-ecology researcher at the Carnegie Institution for Science (Washington, USA). His responses attest that the climate system is complex, and that we need to be practical in dealing with the planet’s ongoing climate emergency.

PhD in atmospheric sciences and professor at Stanford University (USA), Ken Caldeira has pioneered the study of ocean acidification and its impact on coral reefs (1) and geoengineering solutions to mitigate anthropogenic climate change by extracting carbon from the atmosphere and reflecting solar radiation (2, 3). He has also been part of the Intergovernmental Panel on Climate change (IPCC) and assessed zero-emissions scenarios (4, 5). To the right, Ken manoeuvers a drone while collecting aerial data from the Great Barrier Reef in Australia (6). Source.

SARS-Covid-19 is impacting the world. In our home country, Spain, scientists argue that (i) previous budget cuts in public health have weakened our capacity to tackle the pandemic (7), and (ii) the expert panels providing advice to our government should be independent of political agendas in their membership and decisions (8). Nevertheless, the Spanish national and regional governments’ data lack the periodicity, coherence, and detail to harness an effective medical response (9). Sometimes it feels as if politics partly operate by neglecting the science needed to tackle challenges such as the covid pandemic or climate change.

Having said that, even if a country has cultivated and invested in the best science possible, people have difficulties coming to terms with the idea that scientists work with probabilities of alternative scenarios. As much as there are different ways of managing a pandemic, scientists differ about how to mitigate the ecological, economic, and health impacts of a high-carbon society.

Thus, a more and more common approach is to make collective assessments (elicitations) by weighing different points of view across experts — for instance, to establish links between climate change and armed conflict (10) or to evaluate the role of nuclear energy as we transition to a low-carbon energy-production model (11). The overarching goal is to quantify consensus based on different (evidence-based) opinions.

The questions we here ask Ken Caldeira could well have different answers if asked of other experts. Still, as Ken points out, it is urgent that (of the many options available) we use the immense and certainty-proof knowledge we have already about climate change to take actions that work.

Interview done 23 January 2020 

We italicise each question and the name of the person asking the question and cite one to three relevant publications per question. For expanding on Ken Caldeira’s views on climate change, see a sample of his public talks here and here and newspaper articles here and here.

Question 1 of 5: Over recent time, the climate has been gradually warming (particularly in summer), but it is also true that rain and snow are increasing (even outside of the seasons in which precipitation has peaked in the past). If those trends are caused by anthropogenic climate change, can we expect both alternative warming and cooling periods in the future? (12, 13) / Mario Díaz Esteban (conservation scientist)

Caldeira: The broad trend is towards higher temperatures, but there will also be natural weather variability. There will be cold days, but they will on average be a degree or two warmer than they would have been otherwise. There is some evidence that in the northern mid-latitudes that there could be an increase in cold spells. Some scientists think that the waves in the atmosphere that control weather could have higher amplitude bringing warm air from the south farther north and cold air from the north farther south, but this is just a hypothesis at this point and is not well-established fact.

Question 2 of 5: Many authors contend that melting water from the Arctic will distort the Gulf Stream, thereby slowing down or reversing, rather than accelerating, climate warming in Europe. What is the expected impact of climate change on Europe: rapid warming, slow warming, or cooling? (14) / Sergio París Gómez (environmental educator)

Caldeira: In all of the model simulations of which I am aware, a slowing of circulations in the North Atlantic causes Europe to warm more slowly but they do not cause Europe to cool. However, there is a question of how well the models are able to reproduce events in the geologic past, and so the idea that there could be some cooling in some regions cannot be entirely discounted.

Question 3 of 5: Have we already reached a climatic tipping point whereby future reductions in greenhouse-gas emissions might be useless to reverse anthropogenic climate change? (15) / David Vieites Rodríguez (author)

Caldeira: Eliminating greenhouse-gas emissions will not reverse anthropogenic climate change (at least not for many centuries or millennia), but will prevent the Earth from getting a lot warmer. The reason to avoid emissions is not to reverse climate change but to avoid additional warming and the associated climate damage.

Question 4 of 5: Palaeontologists have confirmed that the Earth has experienced five mass extinctions of biodiversity, and we are now witnessing the sixth mass extinction event due to human activities. We also know that our planet has gone through hot and cold periods in the past, so two basic questions to foresee the consequences of ongoing climate change seem to be: How does anthropogenic climate warming fit in the Earth’s history of climate change? And, given that the series of data about past climates are relatively short, can we use such data to project or predict future climate patterns and effects? (16) / Rafael Serra Naranjo (Quercus editor)

Caldeira: The main driver of extinctions over human existence has been changes in how we use the land, and also the hunting of large animals. Climate change has played a relatively small role in extinctions to date. Of course, ocean acidification will destroy coral reefs around the world, and climate zones are moving faster than plants can move. The ice ages did not cause many extinctions but climate change with the waxing and waning of the ice sheets was much slower than we are witnessing today. I think the question of how much climate change and ocean acidification will cause extinctions is an open question, but I do not expect extinction at the scale of the five great extinctions in Earth history.

Question 5 of 5: What should the contribution of nuclear energy versus renewable energies be in a future hypothetically decarbonized society using 0% fossil fuels to produce energy? (17-19) / Salvador Herrando Pérez (author)

Caldeira: If you told me it was year 2100 and we have solved the climate problem and then asked me to guess what happened, I would guess that nuclear power got cheap and that is what allowed us to decarbonise our energy system. However, if we never discovered nuclear energy, it is still within human capacity to develop and deploy technologies that we need to live a modern lifestyle without excessively damaging the environment in which we live. I don’t think we can answer what the role of nuclear should be. The question is what are the most feasible paths to a decarbonised economy, taking into consideration technical, cost, and political hurdles. I am in favour of anything that works.

Acknowledgements: We thank Ken Calderia for kindly accepting to be interviewed and the Carnegie Institution for Science for the photographic material provided. A Spanish version of this article and interview has been published in the December 2020 issue of the magazine Quercus.

Salvador Herrando-Pérez and David R. Vieites


  1. K. Caldeira, M. E. Wickett, Anthropogenic carbon and ocean pH. Nature 425, 365 (2003).
  2. K. Caldeira, G. Bala, L. Cao, The science of geoengineering. Annual Review of Earth and Planetary Sciences 41, 231-256 (2013).
  3. K. Caldeira, G. Bala, Reflecting on 50 years of geoengineering research. Earth’s Future 5, 10-17 (2017).
  4. H. D. Matthews, K. Caldeira, Stabilizing climate requires near-zero emissions. Geophysical Research Letters 35, L04705 (2008).
  5. S. J. Davis et al., Net-zero emissions energy systems. Science 360, eaas9793 (2018).
  6. R. Albright et al., Carbon dioxide addition to coral reef waters suppresses net community calcification. Nature 555, 516-519 (2018).
  7. Editorial, COVID-19 in Spain: a predictable storm? The Lancet Public Health 5, e568 (2020).
  8. A. L. García-Basteiro et al., Evaluation of the COVID-19 response in Spain: principles and requirements. The Lancet Public Health 5, e575 (2020).
  9. S. Trias-Llimós, A. Alustiza, C. Prats, A. Tobias, T. Riffe, The need for detailed COVID-19 data in Spain. The Lancet Public Health 5, e576 (2020).
  10. K. J. Mach et al., Climate as a risk factor for armed conflict. Nature 571, 193-197 (2019).
  11. L. D. Anadón, G. Nemet, E. Verdolini, The future costs of nuclear power using multiple expert elicitations: effects of RD&D and elicitation design. Environmental Research Letters 8, 034020 (2013).
  12. J. Hansen et al., Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous. Atmospheric Chemistry and Physics 16, 3761-3812 (2016).
  13. M. R. Allen, P. A. Stott, J. F. B. Mitchell, R. Schnur, T. L. Delworth, Quantifying the uncertainty in forecasts of anthropogenic climate change. Nature 407, 617-620 (2000).
  14. J. B. Palter, The role of the Gulf Stream in European climate. Annual Review of Marine Science 7, 113-137 (2015).
  15. J. Kreyling, A. Jentsch, C. Beier, Beyond realism in climate change experiments: gradient approaches identify thresholds and tipping points. Ecology Letters 17, 125-e121 (2014).
  16. K. L. Bacon, G. T. Swindles, Could a potential Anthropocene mass extinction define a new geological period? The Anthropocene Review 3, 208-217 (2016).
  17. N. Kopytko, J. Perkins, Climate change, nuclear power, and the adaptation–mitigation dilemma. Energy Policy 39, 318-333 (2011).
  18. K. Shrader-Frechette, What will work: fighting climate change with renewable energy, not nuclear power.  (Oxford University Press, New York, USA, 2012), pp. 1-368.
  19. B. W. Brook, C. J. A. Bradshaw, Key role for nuclear energy in global biodiversity conservation. Conservation Biology 29, 702-712 (2015).




One response

7 02 2021
Citizens ask the experts in climate-change communication |

[…] the second of two consecutive interviews with climate-change experts (see the first one here), readers of the Spanish magazine Quercus have a chat with Katharine Hayhoe. Her words blend […]


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: