The reconstruction of climate in Poland in the past millennium, as measured by several kinds of proxy data, is more complete than that of many other regions in Europe and the world. In fact, the methods of climate reconstruction used here are commonly utilized for other regions. Proxy data available for Poland (whether by documentary, biological, or geothermal evidence) mainly allow for reconstructions of three meteorological variables: air temperature, ground-surface temperature, and precipitation. It must be underlined however, that air temperature reconstructions are possible only for certain times of the year. This is particularly characteristic of biological proxies (e.g., tree rings measure January–April temperature, chironomids provide data for August temperature, chrysophyte cysts identify cold seasons, etc.). Potentially, such limitation has no corresponding documentary evidence. In Poland these data are available only for climate reconstructions covering mainly the last 500 years because the number of historical sources pre-1500 is usually too small. Geothermal data allow for reconstruction of mean annual ground surface temperature generally for the last 500 years. Reconstructions of air temperature that cover the entire, or almost the entire, millennium and have high time resolution are only available from biological proxies (tree rings, chironomids, diatoms, etc.).
At present, the best source of information about climate in Poland in the last millennium is still documentary evidence. This evidence defines a Medieval Warm Period (MWP), which was present in the 11th century and probably ended in the 14th or early 15th century. Air temperature in the MWP was probably about 0.5–1.0°C warmer than contemporary conditions on average, and the climate was characterized by the greatest degree of oceanity throughout the entire millennium. A Little Ice Age (LIA) can be also distinguished in Poland’s climate history. Data show that it clearly began around the mid-16th century and probably ended in the second half of the 19th century. In this LIA, winters were 1.5–3.0°C colder than present conditions, while summers tended to be warmer by about 0.5°C. As a result, the continentality of the climate in the LIA was the greatest for the entire millennium. Mean annual air temperature was probably lower than the modern temperature by about 0.9–1.5°C. The average rise of air temperature since the mid-19th century, which is often called the Contemporary Warming Period (CWP), is equal to about 1°C and is in line with the results of reconstructions using geothermal and dendrochronological methods. The reconstruction of precipitation in Poland is much more uncertain than the reconstruction of air temperature. There was probably considerably higher average precipitation in the 12th century (and particularly in the second half of this century), in the first half of the 16th century, and also in the first half of the 18th century. The second half of the 13th century and the first half of the 19th century were drier than average. In other periods, precipitation conditions were close to average, including for the entire CWP period.
Thomas C. Johnson
The people of East Africa are particularly vulnerable to the whims of their regional climate. A rapidly growing population depends heavily on rain-fed agriculture, and when the rains deviate from normal, creating severe drought or flooding, the toll can be devastating in terms of starvation, disease, and political instability. Humanity depends upon climate models to ascertain how the climate will change in the coming decades, in response to anthropogenic forcing, to better comprehend what lies in store for East African society, and how they might best cope with the circumstances. These climate models are tested for their accuracy by comparing their output of past climate conditions against what we know of how the climate has evolved. East African climate has undergone dramatic change, as indicated by lake shorelines exposed several tens of meters above present lake levels, by seismic reflection profiles in lake basins displaying submerged and buried nearshore sedimentary sequences, and by the fossil and chemical records preserved in lake sediments, which indicate dramatic past change in lake water chemistry and biota, both within the lakes and in their catchments, in response to shifting patterns of rainfall and temperature. This history, on timescales from decades to millennia, and the mechanisms that account for the observed past climate variation, are summarized in this article. The focus of this article is on paleoclimate data and not on climate models, which are discussed thoroughly in an accompanying article in this volume. Very briefly, regional climate variability over the past few centuries has been attributed to shifting patterns of sea surface temperature in the Indian Ocean. The Last Glacial Maximum (LGM) was an arid period throughout most of East Africa, with the exception of the coastal terrain), and the region did not experience much wetter conditions until around 15,000 years ago (15 ka). A brief return to drier times occurred during the Younger Dryas (YD) (12.9–11.7 ka), and then a wet African Humid Period until about 5 ka, after which the region, at least north of Lake Malawi at ~10º S latitude, became relatively dry again. The penultimate ice age was much drier than the LGM, and such megadroughts occurred several times over the previous 1.3 million years. While the African continent north of the equator experienced, on average, progressively drier conditions over the past few million years, unusually wet periods occurred around 2.7–2.5, 1.9–1.7, and 1.1–0.7 million years ago. By contrast, the Lake Malawi basin at ~10º—14º S latitude has undergone a trend of progressively wetter conditions superimposed on a glacial–dry, interglacial–wet cycle since the Mid-Pleistocene Transition at ~900 ka.
David S. G. Thomas
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.
Quaternary paleoclimate reconstructions in tropical-subtropical southern Africa (taken here as approximately south of latitude 17 oS) require both knowledge of the key relevant elements of the atmospheric and climate systems over the subcontinent and a realistic assessment of the possibilities and limitations of the proxy data sources in the region. Orbital forcing and southern hemisphere ocean temperature changes are widely considered as primary drivers of temporal and spatial changes in the relative influence of different components of the circulation system (tropical Indian ocean monsoon, tropical Atlantic moisture, and temperate westerlies) that in turn drive precipitation distributions, amounts, and seasonality. Major debates in recent decades have focused on the timing and extent of aridity and humidity shifts, and the relative contribution of temperate and tropical sources of precipitation during the last approximately 100,000 years, notably at the Last Glacial Maximum (LGM) and during the Holocene climate optimum.
Many of the debates and uncertainties that have emerged are also a function of proxy data sources: where they are located, how they are interpreted, and their resolution. Extrapolation of data from marine core and high resolution terrestrial records to subregions where proxies are sparse, low resolution, or difficult to transform from environmental to climatic signals, may have oversimplified representation of the spatial variability of past climates in a region where variability is a norm today. Particular issues occur in, but are not confined to, the southern African interior, which to date has largely been devoid of reliable precipitation proxies, and where available proxies provide reconstructions of physical changes in landscape systems that can prove difficult to translate to high precision hydrological and rainfall records. Elsewhere, developments in interpreting palynological and isotope records have led to reanalysis of past simple interpretations of hydrological fluxes in the last 50,000 years. Now, a suite of new isotopic proxies derived from previously under-investigated areas or innovative biological and sedimentary sources, and a more realistic interpretation of existing records are generating a suite of testable hypotheses regarding Late Quaternary hydrodynamics. These include establishing the degree of cooling in mountainous regions and clarifying the northerly extent of temperate westerly moisture penetration during cold phases, as well as establishing the contribution of tropical Atlantic moisture to interior wetting and associated feedback mechanisms.
Regional models were originally developed to serve weather forecasting and regional process studies. Typical simulations encompass time periods in the order of days or weeks. Thereafter regional models were also used more and more as regional climate models for longer integrations and climate change downscaling. Regional climate modeling or regional dynamic downscaling, which are used interchangeably, developed as its own branch in climate research since the end of the 1990s out of the need to bridge the obvious inconsistencies at the interface of global climate research and climate impact research. The primary aim of regional downscaling is to provide consistent regional climate change scenarios with relevant spatial resolution to serve detailed climate impact assessments.
Similar to global climate modeling, the early attempts at regional climate modeling were based on uncoupled atmospheric models or stand-alone ocean models, an approach that is still maintained as the most common on the regional scale. However, this approach has some fundamental limitations, since regional air-sea interaction remains unresolved and regional feedbacks are neglected. This is crucial when assessing climate change impacts in the coastal zone or the regional marine environment. To overcome these limitations, regional climate modeling is currently in a transition from uncoupled regional models into coupled atmosphere-ocean models, leading to fully integrated earth system models. Coupled ice-ocean-atmosphere models have been developed during the last decade and are currently robust and well established on the regional scale. Their added value has been demonstrated for regional climate modeling in marine regions, and the importance of regional air-sea interaction became obvious. Coupled atmosphere-ice-ocean models, but also coupled physical-biogeochemical modeling approaches are increasingly used for the marine realm. First attempts to couple these two approaches together with land surface models are underway. Physical coupled atmosphere-ocean modeling is also developing further and first model configurations resolving wave effects at the atmosphere-ocean interface are now available. These new developments now open up for improved regional assessment under broad consideration of local feedbacks and interactions between the regional atmosphere, cryosphere, hydrosphere, and biosphere.
Martin Claussen, Anne Dallmeyer, and Jürgen Bader
There is ample evidence from palaeobotanic and palaeoclimatic reconstructions that during early and mid-Holocene between some 11,700 years (in some regions, a few thousand years earlier) and some 4200 years ago, subtropical North Africa was much more humid and greener than today. This African Humid Period (AHP) was triggered by changes in the orbital forcing, with the climatic precession as the dominant pacemaker. Climate system modeling in the 1990s revealed that orbital forcing alone cannot explain the large changes in the North African summer monsoon and subsequent ecosystem changes in the Sahara. Feedbacks between atmosphere, land surface, and ocean were shown to strongly amplify monsoon and vegetation changes. Forcing and feedbacks have caused changes far larger in amplitude and extent than experienced today in the Sahara and Sahel. Most, if not all, climate system models, however, tend to underestimate the amplitude of past African monsoon changes and the extent of the land-surface changes in the Sahara. Hence, it seems plausible that some feedback processes are not properly described, or are even missing, in the climate system models.
Perhaps even more challenging than explaining the existence of the AHP and the Green Sahara is the interpretation of data that reveal an abrupt termination of the last AHP. Based on climate system modeling and theoretical considerations in the late 1990s, it was proposed that the AHP could have ended, and the Sahara could have expanded, within just a few centuries—that is, much faster than orbital forcing. In 2000, paleo records of terrestrial dust deposition off Mauritania seemingly corroborated the prediction of an abrupt termination. However, with the uncovering of more paleo data, considerable controversy has arisen over the geological evidence of abrupt climate and ecosystem changes. Some records clearly show abrupt changes in some climate and terrestrial parameters, while others do not. Also, climate system modeling provides an ambiguous picture.
The prediction of abrupt climate and ecosystem changes at the end of the AHP is hampered by limitations implicit in the climate system. Because of the ubiquitous climate variability, it is extremely unlikely that individual paleo records and model simulations completely match. They could do so in a statistical sense, that is, if the statistics of a large ensemble of paleo data and of model simulations converge. Likewise, the interpretation regarding the strength of terrestrial feedback from individual records is elusive. Plant diversity, rarely captured in climate system models, can obliterate any abrupt shift between green and desert state. Hence, the strength of climate—vegetation feedback is probably not a universal property of a certain region but depends on the vegetation composition, which can change with time. Because of spatial heterogeneity of the African landscape and the African monsoon circulation, abrupt changes can occur in several, but not all, regions at different times during the transition from the humid mid-Holocene climate to the present-day more arid climate. Abrupt changes in one region can be induced by abrupt changes in other regions, a process sometimes referred to as “induced tipping.” The African monsoon system seems to be prone to fast and potentially abrupt changes, which to understand and to predict remains one of the grand challenges in African climate science.