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Article | WTW Research Network Newsletter

Natural climate variability and our uneven transition to a warmer world

By Scott St. George | June 21, 2023

People are pushing Earth’s climate to behave in ways never seen before. But if we want to truly take the measure of climate change, we must also account for the abiding influence of natural factors.
Aerospace|Climate|Environmental Risks|ESG and Sustainability|Marine|Willis Research Network
Climate Risk and Resilience

Humans are now the dominant force acting upon the climate of our home planet. Since the start of the industrial age, we have loaded the atmosphere with carbon dioxide and other greenhouse gases, reconfigured the way sunlight is absorbed and reflected on land and in the ocean, and pumped dust, smoke and other fine particulates into the stratosphere.[1] At the Earth’s surface our species has had an enormous sway, with construction and agriculture moving ten times the amount of rock and sediment than all natural processes combined[2]. The unintended consequence of this planet-wide transformation is that now the Earth has an energy imbalance[3] — the planet returns less radiation to space than it receives from the sun— and because of that, heat has accumulated continuously over the past decades.

As a result, today we live in a world that is warmer than it has been at any other time in the history of civilization. Human actions have caused the global mean temperature to rise by 0.9°C–1.2°C since the late 19th century, and the most recent decade was, more likely than not, the hottest in at least the past 125,000 years[4]. In the past, our Earth’s climate certainly experienced prolonged shifts to warmer conditions that were due to entirely natural causes[5]. But compared to the current human-induced warming, those earlier climate events were not as hot and much more local in scope[6]. The consequences of our modern warming have cascaded through the environment, pushing global sea level to a modern-day high[7], causing the rapid and near-universal retreat of mountain glaciers[8], and in some places, expanding the wildfire ‘season’ into months that were, in past decades, usually free of fires[9].

In the years to come, the human hand will take an even tighter grip on the wheel. Regardless of our choices about energy use and climate policy, global temperatures are projected to cross the 1.5°C threshold by the early 2030s[10]. It’s virtually certain that, for the global average, sea level will continue to rise through the 21st century and, over the long term, precipitation over land will likewise go up, although we should see contrasting trends from place to place[11]. But don’t expect our future to be a smooth, gradual climb towards a new normal as is sometimes shown in climate projections. Instead, in the real world, climate change will come at us by an uneven transition to warmer, wetter, or drier conditions and will reflect the combined (and sometimes opposing) influences of both human activities and what’s known as natural variability.

Nature turns the climate dials

The familiar maxim that the climate is always changing is certainly true[12]. Massive volcanic eruptions can throw a plume of sulphate aerosols into the upper atmosphere, reflecting more sunlight back to space and causing the Earth to cool for years afterward[13]. The Sun itself is also variable, throwing out a bit more or less energy every 11 years. And very slow variations in the Earth’s orbit nudge the total amount and distribution of the Sun’s energy received at the ground, acting as both a trigger and terminator for the ice ages[14]. Solar variations and volcanic eruptions are the two main types of natural radiative forcings, factors beyond our control that affect the amount of energy that enters and leaves the Earth’s atmosphere.

On top of that, even without any major tweaks to its overall energy budget, our climate can still wander up and down entirely on its own. The components of the Earth’s climate system — the oceans, the atmosphere, sea ice, the biosphere — constantly act and interact with each other. In the same way that a build-up of friction creates static electricity, as energy moves from one component to another, the system’s internal variability makes change between our day-to-day weather and prolonged shifts lasting for several decades[15].

When the easterly winds that cut across the Pacific Ocean begin to slacken, the warmest water in the world moves eastward away from Indonesia and towards the Galapagos Islands. That temporary relocation upsets the normal pattern of air and ocean currents, causing the planet-wide climate disruption called El Niño[16]. The tropical Pacific can also combine with Japan’s Kuroshio Current and the semi-permanent low-pressure system that sits over the Gulf of Alaska to produce the North Pacific Oscillation, a regional seesaw that switches between warm and cold conditions roughly every 10 to 15 years[17]. And in the North Atlantic, winter storms dance between a low-pressure system near Iceland and a high-pressure system near the Azores Islands. When North Atlantic Oscillation Index is high, the contrast between these two systems is strong, the British Isles and northern Europe are hammered by wet weather, but the Mediterranean is left cold and dry[18].

Wild climates of the past

Today a constellation of satellites allows us to witness change anywhere on the Earth’s surface from one sunrise to the next. If we want to know how temperature, rainfall, or other key climate metrics can behave over many years or decades, we can comb through weather records stretching back to the middle of the 1800s (or in a few rare cases, even earlier). But even a century or two is nowhere near enough time to fully take the measure of Earth’s climate. Fortunately, we can turn to unconventional sources of information — ancient trees, muddy lake bottoms, underground cave deposits, and many others — to extend our perspective much farther back in time. And those geological or biological records have proven, many times over, that natural variability can, all by itself, produce spectacular changes very different from our recent experience.

As chronicled in The Prose Edda, the twilight of the Norse gods is preceded by the Fimbulvinter (great winter) “when snow drives from all quarters, the frosts are so severe, the winds so keen and piercing, that there is no joy in the sun”[19]. The Fimbulvinter could have been inspired by a real global catastrophe, as the Nordic countries, and indeed much of the rest of the world, did experience a protracted environmental crisis early in the 6th Century. The widespread turmoil is thought to have been triggered by two massive eruptions: the first in 535 CE or early 536 CE from an unknown but likely North American volcano[20] and the second in 539 or 540 CE from Ilopango in present-day El Salvador[21]. Together these two eruptions dropped summer temperatures in Europe by 1.6–2.5°C[22] and served as the inciting incident for the Late Antique Little Ice Age[23], a prolonged cold phase lasting more than a century.

Over the past two decades, California and its neighboring states have been so dry for so long that scientists have referred to the ongoing situation not as a drought but as a ‘megadrought’. The 21st century megadrought is the product of natural variability stacked on top of human-caused climate change[24], but tree rings show the region has been plagued by similar events in the past solely due to natural causes[25]. The fact that megadroughts can happen even without climate change — simply due to a string of bad luck — means that we are very likely underestimating their true risks, both in the western United States and elsewhere[26].

The grand climate tug-of-war

Writing in 1901, the Oxford geographer Andrew Herbertson conjured the oft-repeated phrase “Climate is what on an average we may expect, weather is what we actually get[27]”. More than a century later, the American climate scientist Clara Deser proposed this adage be rephrased to reflect our current situation. In her version, anthropogenic climate change is what we expect. But what we’ll actually get is anthropogenic climate change plus natural climate variability[28].

As we look towards the future, the balance between those two factors will depend on scale — both geographic scale and time scale[29]. If we average conditions over the entire planet, the fingerprint of humans on the climate is much more evident. But as we zoom down to more and more local scales — a single country, state or city, for instance — the relative contribution of internal variability becomes more important. And although by the end of the 21st century the human factor will eventually win out, in the near and intermediate term (the next 20 to 40 years) the contribution from natural variability will be just as large or larger.

Making things even more complicated, those systems that create natural variability are themselves part of Earth’s climate and are also affected by us. Even the El Nińo-Southern Oscillation — the chief cause of year-to-year shifts in global climate — is expected to change its ways as the tropical Pacific Ocean heats up[30]. But scientists aren’t sure exactly how ENSO and many other aspects of natural variability will play out in a warmer world[31]. Organizations needs to build that uncertainty into risk scenarios to correctly anticipate future climate change or climate-related perils.

Bringing balance to our climate futures

All of us will live under a global climate unlike the past. But if we want to understand the full span of possible future climates and their effects on us, our business, or our clients, we need to account for both human and natural factors. Recently, the WTW Research Network has struck new partnerships with global experts in climate and Earth Science so we can understand the likely influence of natural variability now and in the future. Dr. Simon Mason at Columbia University (New York) is engaged with WTW to upgrade our Climate Quantified™ platform with state-of-the-art climate scenarios that also include natural variability. We’ve also partnered with Prof. Markus Stoffel at the University of Geneva to bring to light the systemic risk to global society — particularly agriculture — posed by the next massive volcanic eruption. And we’re cultivating new collaborations so seasonal forecasts of El Nińo and other recurring climate patterns can be used to prepare for natural hazards or resource shortfalls several months in advance.

Through our own actions, we’ve already rearranged the magnificent puzzle that is Earth’s climate. And from this point onwards, the human imprint on the picture will only get more and more obvious. But all the pieces matter. Knowing how natural variability and human-induced changes will act together will provide us with a more realistic foundation to identify and respond to climate-related risks and opportunities.


  1. Forster et al. (2021), The Earth’s energy budget, climate feedbacks, and climate sensitivity. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 923–1054. Return to article
  2. Wilkinson (2005), Humans as geologic agents: A deep-time perspective, Geology 33 (3): 161-164. Return to article
  3. von Schuckmann et al. (2023), Heat stored in the Earth system 1960–2020: Where does the energy go? Earth System Science Data 15: 1675-1709. Return to article
  4. Gulev et al. (2021), Changing state of the climate system. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 287–422. Return to article
  5. Neukom et al. (2019), No evidence for globally coherent warm and cold periods over the preindustrial Common Era, Nature 571: 550–554. Return to article
  6. St. George (2019), Aberrant synchrony of present-day warming, Nature 571: 484-485. Return to article
  7. Dangendorf et al. (2019), Persistent acceleration in global sea-level rise since the 1960s, Nature Climate Change 9: 705-710. Return to article
  8. Solomina et al. (2015), Holocene glacier fluctuations, Quaternary Science Reviews 111: 9-34. Return to article
  9. Swain (2021), A shorter, sharper rainy season amplifies California wildfire risk, Geophysical Research Letters 48 (5),e2021GL092843. Return to article
  10. Lee, J.-Y., J. Marotzke, G. Bala, L. Cao, S. Corti, J.P. Dunne, F. Engelbrecht, E. Fischer, J.C. Fyfe, C. Jones, A. Maycock, J. Mutemi, O. Ndiaye, S. Panickal, and T. Zhou, 2021: Future Global Climate: Scenario-Based Projections and Near- Term Information. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 553–672. Return to article
  11. ibid. Return to article
  12. Mitchell (1976), An overview of climate variability and its causal mechanisms. Quaternary Research 6: 481-493. Return to article
  13. Oppenheimer (2003), Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia) 1815. Progress in Physical Geography 27: 230-259. Return to article
  14. Berger (1988), Milankovitch Theory and climate. Reviews of Geophysics 26: 624-657. Return to article
  15. Deser et al. (2012), Uncertainty in climate change projections: The role of internal variability. Climate Dynamics 38: 527-546. Return to article
  16. Timmerman et al. (2018), El Niño–Southern Oscillation complexity. Nature 559: 535-545. Return to article
  17. Newman et al. (2016), The Pacific Decadal Oscillation, revisited. Journal of Climate 29: 4399-4427. Return to article
  18. Lindsey and Dahlman (2009), Climate Variability: North Atlantic Oscillation. Return to article
  19. Anderson (1879), The Younger Edda: Also Called Snorre’s Edda, or the Prose Edda, S.C. Griggs and Company, Chicago, 217 p. Return to article
  20. Sigl et al. (2015), Timing and climate forcing of volcanic eruptions for the past 2,500 years. Nature 523: 543-549. Return to article
  21. Dull et al. (2019), Radiocarbon and geologic evidence reveal Ilopango volcano as source of the colossal ‘mystery’ eruption of 539/40 CE. Quaternary Science Reviews 222: 105855. Return to article
  22. PAGES 2k Consortium (2013), Continental-scale temperature variability during the past two millennia. Nature Geoscience 6: 339–346. Return to article
  23. Büntgen et al. (2016), Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD. Nature Geoscience 9: 231-236. Return to article
  24. Williams et al. (2020), Large contribution from anthropogenic warming to an emerging North American megadrought. Science 368: 314-318. Return to article
  25. Cook et al. (2022), Megadroughts in the Common Era and the Anthropocene. Nature Reviews Earth & Environment 3: 741-757. Return to article
  26. Ault and St. George (2018), Unraveling the mysteries of megadrought. Physics Today 8: 44-50. Return to article
  27. Herbertson (1901), Outlines of Physiography: An introduction to the study of the Earth. Edward Arnold (London), 312 p. Return to article
  28. Deser (2020), “Certain uncertainty: The role of internal climate variability in projections of regional climate change and risk management”. Earth’s Future 8, e2020EF001854. Return to article
  29. Lehner et al. (2020), Partitioning climate projection uncertainty with multiple large ensembles and CMIP5/6. Earth System Dynamics 11, 491-508. Return to article
  30. Lee et al. (2023), How the pattern of trends across the tropical Pacific Ocean is critical for understanding the future climate. ENSO Blog, Return to article
  31. Sobel (2023), How El Niño may test the limits of our climate knowledge. TIME, March 17, 2023. Return to article

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WTW Research Network, WTW
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