Climate variability and change

• 1. Introduction

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  Climate variability and change is one of the most studied phenomena in Earth science, yet it encompasses everything from a single storm season to shifts that take millions of years to unfold. When scientists talk about climate variability, they mean any variation lasting longer than individual weather events. When they say climate change, they mean variations that persist for decades or more. The distinction matters, but the two are deeply linked.

  Since the Industrial Revolution, human activity has become a force shaping that system. Greenhouse gases, land use, aerosols, deforestation: all of these have tilted the energy balance of an entire planet. But long before humans arrived, the climate had been churning through cycles, oscillations, and abrupt lurches driven by the Sun, volcanic eruptions, wobbling orbits, and the slow drift of continents.

  Half of the 2021 Nobel Prize in Physics went to Klaus Hasselmann for his work on stochastic resonance in climate systems, jointly with Syukuro Manabe. That a prize for physics should hinge on understanding how random weather noise can produce long-range climate swings tells you something about how strange and layered this subject really is. The chapters ahead move through the forces at work, the traces they leave behind, and the consequences they carry for life on Earth.

• 2. The Energy Budget

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  Earth's climate receives nearly all of its energy from the Sun, then radiates that energy back to outer space. The balance between incoming and outgoing energy is what climatologists call Earth's energy budget. When more energy arrives than leaves, the budget is positive and the planet warms. When more leaves than arrives, the budget is negative and the planet cools.

  That energy does not stay put. Winds, ocean currents, and other mechanisms shuttle heat around the globe, producing the range of climates humans have come to think of as normal. The long-term average of weather conditions in a given region is what gives that region its climate.

  Not all forces that shape climate come from inside the system. External forcings can push the system from outside: changes in solar radiation, shifts in Earth's orbital geometry, and volcanic eruptions all qualify. Internal forcings, by contrast, come from the redistribution of energy already inside the system, such as changes in ocean circulation patterns like the thermohaline circulation. Some of these feedbacks amplify the original signal; others dampen it. And when certain thresholds are crossed, the change can become rapid or irreversible.

• 3. Oscillations and Internal Rhythms

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  El Nino, formally the El Nino-Southern Oscillation or ENSO, is the most prominent known source of year-to-year variability in weather and climate worldwide. Its cycle runs every two to seven years, with the warmer El Nino phase lasting nine months to two years within that longer pattern. The cold tongue of the equatorial Pacific is not warming as fast as the rest of the ocean, partly because of increased upwelling of cold water off the west coast of South America.

  Beyond ENSO, a range of other oscillations operate on different timescales. The Madden-Julian oscillation moves as an eastward-traveling band of increased rainfall across the tropics over a period of 30 to 60 days. The Quasi-biennial oscillation shifts the dominant stratospheric wind above the equator from easterly to westerly and back over a period of 28 months. The Atlantic multidecadal oscillation runs on a cycle of roughly 55 to 70 years, influencing rainfall, droughts, and the frequency and intensity of hurricanes.

  All of these are described as quasiperiodic: they recur, but not with clockwork regularity. That irregularity is part of what makes isolating individual climate cycles so difficult. Many climate changes have both a random aspect and a cyclical aspect, a behavior Klaus Hasselmann and colleagues named stochastic resonance: the inertia of oceans and glaciers can amplify random weather noise into longer, larger oscillations.

• 4. What Drives Change From Outside

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  Variations in Earth's orbit produce what are called Milankovitch cycles, named for their three components: changes in the eccentricity of Earth's path around the Sun, shifts in the tilt of Earth's rotational axis, and the slow precession of that axis. These orbital changes redistribute sunlight across latitudes and seasons without much changing the total amount arriving. Their fingerprints appear in the stratigraphic record, in the advance and retreat of the Sahara, and in the rhythm of glacial and interglacial periods.

  Three to four billion years ago, the Sun emitted only 75 percent as much power as it does today. Given that output, liquid water should not have existed on Earth's surface if the atmosphere had the same composition it has now. Yet evidence from the Hadean and Archean eons confirms that water was there, a puzzle that scientists call the faint young Sun paradox. One leading hypothesis is that greenhouse gas concentrations in the early atmosphere were far higher than today.

  Volcanic eruptions large enough to affect climate are those that inject more than 100,000 tons of sulfur dioxide into the stratosphere. The sulfate aerosols and SO2 scatter or absorb solar radiation, forming a global haze of sulfuric acid that cools the surface for years. The 1991 eruption of Mount Pinatubo lowered global temperatures by about 0.5 degrees Celsius for up to three years. The 1815 eruption of Mount Tambora produced what became known as the Year Without a Summer. At a longer scale, large igneous province eruptions occur a few times every 50 million to 100 million years and can flood the surface with volcanic rock while releasing carbon dioxide stored in the mantle.

• 5. Life as a Climate Force

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  Roughly 2.3 billion years ago, the evolution of oxygenic photosynthesis triggered a glaciation by stripping the atmosphere of carbon dioxide, then a dominant greenhouse gas, and introducing free oxygen. Life was not a passive bystander to climate; it was rewriting the atmosphere.

  About 300 million years ago, a second glaciation was ushered in by the long-term burial of decomposition-resistant plant material from vascular land plants. Those buried forests became coal, drawing carbon out of the atmosphere and forming a carbon sink large enough to reshape global temperatures. Fifty-five million years ago, flourishing marine phytoplankton helped bring the Paleocene-Eocene Thermal Maximum to an end. Forty-nine million years ago, 800,000 years of blooms by arctic azolla ferns reversed a global warming trend. And over the past 40 million years, the spread of grass-grazer ecosystems has driven a general global cooling.

  Today, climate changes cascade back into ecosystems. A warming region can shift flowering and fruiting times earlier, pushing the life cycles of the organisms that depend on those plants out of alignment. Sudden or large changes can devastate ecosystems: the Carboniferous Rainforest Collapse, an extinction event 300 million years ago, abruptly fragmented vast tropical rainforests that once covered equatorial Europe and America into isolated islands of habitat.

• 6. Reading the Past

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  Paleoclimatology uses physical and biological archives to reconstruct climates that predate any thermometer. Ice cores drilled from the Antarctic ice sheet preserve air bubbles that contain the actual atmosphere of thousands of years ago, including its carbon dioxide concentrations. The 18O/16O ratio in calcite and ice samples gives scientists a window into past ocean temperatures.

  Tree rings record stress. Too little rain or unsuitable temperatures slow a tree's growth, and scientists can read that in the ring width. This field is called dendroclimatology. Pollen grains are even more durable: the outer surface is made of a resilient material that resists decay, so layers of pollen in sediment tell researchers which plant communities occupied a region across thousands of years. Pollen studies have tracked vegetation shifts throughout the Quaternary glaciations and especially since the last glacial maximum.

  Beetle remains in freshwater and land sediments offer yet another window. Different beetle species inhabit different climatic conditions, and because beetle genetics have changed very little over millennia, the species found in ancient sediment can be matched against their known present-day climatic ranges to infer past conditions. Historical climatology adds a different layer by pulling from written sources: sagas, chronicles, maps, local histories, paintings, drawings, and rock art. Archaeological evidence and oral history also reveal how climate shifts reshaped settlement and agricultural patterns, and changes in climate have been linked to the rise and collapse of multiple civilizations.

• 7. Modern Warming and What Comes Next

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  Land ice sheets in both Antarctica and Greenland have been losing mass since 2002, with an acceleration in that loss recorded after 2009. Global sea levels have been rising as thermal expansion and ice melt combine. The oceans have absorbed about 90 percent of excess heat, which has caused land surface temperatures to climb faster than sea surface temperatures. The Northern Hemisphere, with its larger landmass-to-ocean ratio, has seen greater average temperature increases than the Southern Hemisphere.

  The upper atmosphere has been cooling at the same time that the lower atmosphere warms, a pattern that confirms the greenhouse effect is at work rather than some solar-driven explanation. Solar cycles cannot account for the warming observed from the 1980s onward. The opening of the Northwest Passage and recent record low Arctic ice extents have not been seen for at least several centuries; early explorers were unable to make an Arctic crossing even in summer.

  During the Last Glacial Maximum roughly 25,000 years ago, sea levels were about 130 meters lower than today. In the early Pliocene, global temperatures were only 1-2 degrees Celsius warmer than the present, yet sea levels stood 15-25 meters higher. That gap between temperature and sea level response reflects how much ice remains to be accounted for, and researchers studying regional tipping points such as rainforest loss, ice sheet dynamics, and permafrost thaw are now investigating whether a cascade of such tipping points could accelerate change beyond what any single forcing alone would predict.