A core sample of ancient Antarctic ice that was extracted from over three kilometers below the surface is kept in a laboratory in Bern, Switzerland. It contains trapped air bubbles from a time when the environment was radically different from what it is today. By examining those bubbles, scientists are able to decipher hundreds of thousands of years of climate history from the concentration of gases and the ratio of oxygen isotopes preserved in the frozen record.
The climate of Earth has never been steady, as those cores demonstrate. It has fluctuated between ice ages and warm spells, volcanic winters and prolonged warmth, and changes brought about by orbital mechanics, ocean circulation, and the gradual shift of continents. Long before there was anything like contemporary human civilization, the planet was conducting its own climate experiments. Knowing what caused those shifts is not only fascinating historically, but it is also the only way to comprehend why the current situation differs.
The most potent natural drivers of climate change are almost imperceptible over the course of a human lifetime. The ice age cycles that have characterized much of the last few million years of Earth’s history are caused by Milankovitch cycles, which are the slow, predictable wobbles in Earth’s orbit and axial tilt that occur over tens of thousands to hundreds of thousands of years. Over millennia, ice sheets expand or contract as a result of changes in the orbital geometry that affect how much sunlight reaches the Northern Hemisphere.
Milutin Milanković’s mathematical calculations of orbital mechanics in the early 20th century, which climate scientists have since verified with the ice core record, contributed to the current interglacial period, the comparatively warm period that allowed agriculture and civilization to flourish. These cycles continue to function. They are just too sluggish to explain the warming seen over the previous 70 years, and they are currently pointing in a very modest cooling direction.
Volcanic eruptions occur simultaneously in two opposing directions on a timescale that is more perceptible to humans. When Mount Pinatubo erupted in the Philippines in June 1991, it released enough sulfur dioxide into the stratosphere to create a layer of reflective aerosol that lowered average global temperatures by about 0.5°C for approximately a year and a half. This cooling effect can be seen in records of global temperatures as well as in photos of unusually vibrant sunsets that summer.
The short-term mechanism is that. However, during geological time, prolonged periods of intensive volcanism have been linked to warming episodes that took millions of years to develop, and volcanic activity has been a significant source of atmospheric carbon dioxide. Depending on the clock you’re using, the volcano can act as a warmer or cooler.
Since the data on solar variation is exceptionally clear, it is worthwhile to directly address this natural factor, which is the one most commonly cited in public discourse regarding the causes of the current warming. Since 1978, solar output has been continuously measured by satellite, and there has been no net growth over that time.
The Maunder Minimum, a period of exceptionally low sunspot activity during the 17th century, is linked to the Little Ice Age, a colder period in North American and European history. Climate has been impacted by solar fluctuation in the past. That is just not the current situation. It is not supported by the measurements, and the pattern of global warming is different from what solar forcing would result in.
On periods of years to decades, El Niño and La Niña, as well as longer-cycle phenomena like the North Atlantic Oscillation, can drastically alter regional temperatures by redistributing heat within the climate system. Globally, El Niño years are typically warmer than La Niña years. However, rather than adding heat to the system, these are redistributions.
On top of an already warming baseline, they explain why 2023 was unusually warm, but not the baseline itself. When all five natural causes are considered collectively, it is difficult to ignore the fact that they either operate too slowly, in the wrong direction, or with insufficient magnitude to account for a warming trend that has accelerated noticeably since about 1950, coinciding with the start of industrial fossil fuel combustion. The powers of nature still exist. Simply put, they are not responsible for the present course.
