Holocene Atmospheric Temperature Changes and the Phenomenon of Global Warming

Before examining the warming driven by human activity, a fundamental question must first be addressed: how did Earth's average temperature change during the past ten thousand years? The period following the end of the last glacial period is known as the Holocene. Because it represents the most recent interglacial interval, many researchers use it as a reference framework for evaluating how unusual modern industrial-era warming may be. Numerous studies have pointed to a pronounced interval of global warmth during the early to middle Holocene. Yet many climate models simulate the opposite trend. This disagreement between empirical data and simulations has become known as the Holocene temperature conundrum.

To understand this puzzle, one must begin with the natural archives that record past temperature. Scientists reconstruct ancient climates using proxy records, which rely on indirect indicators preserved in natural materials that accumulate over time. Ice cores, marine sediments, biochemical markers, pollen assemblages, and many other biological and physical traces provide evidence that can be translated into estimates of past temperature. By integrating nearly seven hundred records from both land and ocean environments, researchers have reconstructed the Holocene global mean surface temperature (GMST). These reconstructions indicate that around 6,500 years ago there was a several-century interval of relative warmth. During this time, global temperatures were approximately 0.6 to 0.7°C higher than those of the nineteenth century, with the highest estimates approaching about 1.8°C. Evidence from subsurface ocean temperatures supports this interpretation. Records from equatorial and subtropical regions of the Pacific and Atlantic Oceans show that early Holocene waters were warmer than those of the later Holocene. Analyses of geothermal heat flow in boreholes also suggest that between roughly 6,000 and 7,000 years ago, average surface temperatures were about 1 to 2°C higher than the modern baseline defined by the period 1960 to 1991.

Holocene temperature changes（圖片來源：Dragons flight，採用 CC BY-SA 3.0 授權）

Glacial landscapes provide another independent line of evidence. Mountain glaciers across the world retreated to their smallest extent between about 10,000 and 6,000 years ago, consistent with a relatively warm climate. Later, between roughly 5,000 and 3,000 years ago, glaciers in both the Northern and Southern Hemispheres began to advance again. This expansion marks the onset of the neoglacial period during the middle to late Holocene. Large ice sheets show a similar pattern. Both the Greenland Ice Sheet and the West Antarctic Ice Sheet appear to have gradually thickened during the later Holocene. When these glacial records are combined with temperature proxies, the overall pattern consistently suggests that the early half of the Holocene contained a globally warmer interval.

If such a warming peak truly occurred, what mechanisms drove it? The answer lies largely in variations in Earth's orbital geometry and the resulting distribution of solar radiation. During the early Holocene, Earth's axial tilt was slightly larger than today, approximately 24° to 24.2°. At the same time, axial precession caused Northern Hemisphere summers to occur closer to perihelion, the point in Earth's orbit closest to the Sun. These orbital conditions increased summer solar radiation at high northern latitudes, producing longer and more intense summers. Greater solar energy allowed snow and ice to melt more easily, enabling forests to expand northward. As vegetation spread and snow cover decreased, the surface absorbed more sunlight. Reduced sea-ice cover further enhanced this effect by allowing ocean waters to release additional heat into the atmosphere. These interacting processes amplified the initial warming.

Vegetation patterns during the mid-Holocene also differed markedly from those of later periods. For example, large regions of the Sahara that are now desert supported extensive vegetation during that time. Pollen records suggest that this expanded vegetation reduced surface albedo, allowing the land to absorb more solar energy. The result was increased summer and autumn temperatures, especially at higher latitudes. When such vegetation changes are incorporated into climate simulations, global mean temperatures around 6,000 years ago can appear about 0.4 to 0.9°C warmer than during the preindustrial era, consistent with proxy-based estimates of the Holocene warming peak. At the same time, atmospheric dust concentrations were lower, which reduced the scattering of sunlight and contributed additional warming. In contrast, greenhouse gas concentrations changed very little during this period. Atmospheric carbon dioxide and methane levels in the mid-Holocene differed only slightly from those of the preindustrial atmosphere.

Climate records from many parts of the world also reveal a gradual cooling trend over the past two thousand years. This cooling ultimately reached its lowest point during the Little Ice Age, which occurred roughly between the fifteenth and nineteenth centuries. The long-term cooling trend corresponds closely with vegetation retreat and the expansion of sea ice and glaciers.

Temperature changes over the past 2,000 years（圖片來源：Dragons flight，採用 CC BY-SA 3.0 授權）

If the natural climate system had already been drifting toward cooler conditions, then the rapid temperature rise that followed the Industrial Revolution becomes even more striking. Against a background of long-term cooling, the sharp warming observed since the nineteenth century highlights the extraordinary magnitude of anthropogenic climate change and clarifies the distinction between natural climatic variability and human influence.

Author: Shui-Ye You

Reference:

Kaufman DS and Broadman E. (2023). Revisiting the Holocene global temperature conundrum. Nature.