Regional changes during the Holocene
To a large degree, the societal impacts of climate change do not depend on global means; regional- and local-scale change is what matters from a societal perspective. As such, even in the study of past climate change, it is critical to examine the spatial heterogeneity of the expressions of global climate change. Paleoclimate proxy data is often quite limited, and large error bars preclude identifying subtle but important spatial patterns in climate variability. However, regional reconstructions from the Holocene are robust enough to provide insights into these spatial variations.
Figure 5.11 below shows some of this heterogeneity in the Mid-Holocene warm period, and also highlights that models, often limited by low spatial resolution, fail to capture the magnitude of spatial trends. Note, for example, a discrepency between data-driven reconstructions for southernmost Africa (showing warming of >4ºC) and modeled temperature anomalies for the same region (showing slight cooling). Both paleoclimate datasets and models have limitations, and it is generally unambiguous which is more accurate. Increasing the spatial distribution of paleoclimate proxy records and the resolution of paleoclimate models will improve the accuracy and precision of regional climate records.
Another important example of regional change is in the cryosphere. Trends in sea ice and glacier extent during the Holocene vary significantly on the century- to millennial-scale, and vary significantly from region-to-region. Specifically, glaciers in the Northern Hemisphere have gradually increased in dimension during the latter half of the Holocene, while glaciers in the Southern Hemisphere have generally decreased. This difference is due to orbital variations resulting in decreasing Northern Hemisphere insolation over this period.
Figure 5.11: Comparison of model (bottom two maps) and data (top two maps) reconstructions of temperature anomalies for the Mid-Holocene (~6,000 years ago). MTCO (two left-hand maps) is the mean temperature of the coldest month, and MTWA (two right-hand maps) is the mean temperature of the warmest month. Temperatures are reported as anomalies relative to modern.