Jonathan Holmes and Philipp Hoelzmann
From the end of the last glacial stage until the mid-Holocene, large areas of arid and semi-arid North Africa were much wetter than present, during the interval that is known as the African Humid Period (AHP). During this time, large areas were characterized by a marked increase in precipitation, an expansion of lakes, river systems, and wetlands, and the spread of grassland, shrub land, and woodland vegetation into areas that are currently much drier. Simulations with climate models indicate that the AHP was the result of orbitally forced increase in northern hemisphere summer insolation, which caused the intensification and northward expansion of the boreal summer monsoon. However, feedbacks from ocean circulation, land-surface cover, and greenhouse gases were probably also important.
Lake basins and their sediment archives have provided important information about climate during the AHP, including the overall increases in precipitation and in rates, trajectories, and spatial variations in change at the beginning and the end of the interval. The general pattern is one of apparently synchronous onset of the AHP at the start of the Bølling-Allerød interstadial around 14,700 years ago, although wet conditions were interrupted by aridity during the Younger Dryas stadial. Wetter conditions returned at the start of the Holocene around 11,700 years ago covering much of North Africa and extended into parts of the southern hemisphere, including southeastern Equatorial Africa. During this time, the expansion of lakes and of grassland or shrub land vegetation over the area that is now the Sahara desert, was especially marked. Increasing aridity through the mid-Holocene, associated with a reduction in northern hemisphere summer insolation, brought about the end of the AHP by around 5000–4000 years before present. The degree to which this end was abrupt or gradual and geographically synchronous or time transgressive, remains open to debate. Taken as a whole, the lake sediment records do not support rapid and synchronous declines in precipitation and vegetation across the whole of North Africa, as some model experiments and other palaeoclimate archives have suggested. Lake sediments from basins that desiccated during the mid-Holocene may have been deflated, thus providing a misleading picture of rapid change. Moreover, different proxies of climate or environment may respond in contrasting ways to the same changes in climate. Despite this, there is evidence of rapid (within a few hundred years) termination to the AHP in some regions, with clear signs of a time-transgressive response both north to south and east to west, pointing to complex controls over the mid-Holocene drying of North Africa.
Mineral dust is the most important natural aerosol type by mass, with northern Africa the most prominent source region worldwide. Dust particles are lifted into the atmosphere by strong winds over arid or semiarid soils through a range of emission mechanisms, the most important of which is saltation. Dust particles are mixed vertically by turbulent eddies in the desert boundary layer (up to 6km) or even higher by convective and frontal circulations. The meteorological systems that generate winds strong enough for dust mobilization cover scales from dust devils (~100m) to large dust outbreaks related to low- and high-pressure systems over subtropical northern Africa (thousands of kilometers) and include prominent atmospheric features such as the morning breakdown of low-level jets forming in the stable nighttime boundary layer and cold pools emanating from deep convective systems (so-called haboobs). Dust particles are transported in considerable amounts from northern Africa to remote regions such as the Americas and Europe. The removal of dust particles from the atmosphere occurs through gravitational settling, molecular and turbulent diffusion (dry deposition), as well as in-cloud and sub-cloud scavenging (wet deposition). Advances in satellite technology and numerical dust models (including operational weather prediction systems) have led to considerable progress in quantifying the temporal and spatial variability of dust from Africa, but large uncertainties remain for practically all stages of the dust cycle. The annual cycle of dustiness is dominated by the seasonal shift of rains associated with the West African monsoon and the Mediterranean storm track. In summer, maximum dust loadings are observed over Mauritania and Mali, and the main export is directed toward the Caribbean Sea, creating the so-called elevated Saharan Air Layer. In winter the northeasterly harmattan winds transport dust to the tropical Atlantic and across to southern America, usually in a shallower layer.
Mineral dust has a multitude of impacts on climate and weather systems but also on humans (air pollution, visibility, erosion). Nutrients contained in dust fertilize marine and terrestrial ecosystems and therefore impact the global carbon cycle. Dust affects the energy budget directly through interactions with short- and long-wave radiation, with details depending crucially on particle size, shape, and chemical composition. Mineral dust particles are the most important ice-nuclei worldwide and can also serve as condensation nuclei in liquid clouds, but details are not well understood. The resulting modifications to cloud characteristics and precipitation can again affect the energy (and water) budget. Complicated responses and feedbacks on atmospheric dynamics are known, including impacts on regional-scale circulations, sea-surface temperatures, surface fluxes and boundary layer mixing, vertical stability, near-surface winds, soil moisture, and vegetation (and therefore again dust emission). A prominent example of such complex interactions is the anti-correlation between African dust and Atlantic hurricane activity from weekly to decadal timescales, the causes of which remain difficult to disentangle. Particularly in the early 21st century, research on African dust intensified substantially and became more interdisciplinary, leading to some significant advances in our understanding of this fascinating and multifaceted element of the Earth system.
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.
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.
Dramatic climate changes have occurred in the Baltic Sea region caused by changes in orbital movement in the earth–sun system and the melting of the Fennoscandian Ice Sheet. Added to these longer-term changes, changes have occurred at all timescales, caused mainly by variations in large-scale atmospheric pressure systems due to competition between the meandering midlatitude low-pressure systems and high-pressure systems. Here we follow the development of climate science of the Baltic Sea since observations started in the 18th century to the present. The question of why the water level is sinking around the Baltic Sea coasts could not be answered until the idea of postglacial uplift and the thermal history of the earth were better understood in the 19th century and periodic behavior in time series attracted scientific interest. Herring and sardine fishing successes and failures have led to investigations of fishery and climate change and to the realization that fisheries themselves have strongly negative effects on the marine environment, calling for international assessment efforts. Scientists later introduced the concept of regime shifts when interpreting their data, attributing these to various causes. The increasing amount of anoxic deep water in the Baltic Sea and anthropogenic eutrophication have prompted debate about what is natural and what is anthropogenic, and the scientific outcome of these debates now forms the basis of international management efforts to reduce nutrient leakage from land. The observed increase in atmospheric CO2 and its effects on global warming have focused the climate debate on trends and generated a series of international and regional assessments and research programs that have greatly improved our understanding of climate and environmental changes, bolstering the efforts of earth system science, in which both climate and environmental factors are analyzed together.
In regions such as the Baltic Sea, attributing the causes of climate variability and change has been hampered by the spatial and temporal limitations of observations and by an incomplete understanding of driving mechanisms, leaving room for speculation as to both the reasons for changes and the role of the climate in them. Major achievements of past centuries were developing and organizing regular observation and monitoring programs. The free availability of data sets has supported the development of more accurate forcing functions for Baltic Sea models and made it possible to better model the Baltic Sea–North Sea system, including the development of coupled land–sea–atmosphere models. Most indirect and direct observations of the climate find great variability and stochastic behavior, so conclusions based on short time series are problematic, leading to qualifications about periodicity, trends, and regime shifts. Starting in the 1980s, systematic research into climate change has considerably improved our understanding of regional warming and multiple threats to the Baltic Sea. Several aspects of regional climate and environmental changes and how they interact are, however, unknown and merit future research.
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.
An orbitally induced increase in summer insolation during the last glacial-interglacial transition enhanced the thermal contrast between land and sea, with land masses heating up compared to the adjacent ocean surface. In North Africa, warmer land surfaces created a low-pressure zone, driving the northward penetration of monsoonal rains originating from the Atlantic Ocean. As a consequence, regions today among the driest of the world were covered by permanent and deep freshwater lakes, some of them being exceptionally large, such as the “Mega” Lake Chad, which covered some 400 000 square kilometers. A dense network of rivers developed.
What were the consequences of this climate change on plant distribution and biodiversity? Pollen grains that accumulated over time in lake sediments are useful tools to reconstruct past vegetation assemblages since they are extremely resistant to decay and are produced in great quantities. In addition, their morphological character allows the determination of most plant families and genera.
In response to the postglacial humidity increase, tropical taxa that survived as strongly reduced populations during the last glacial period spread widely, shifting latitudes or elevations, expanding population size, or both. In the Saharan desert, pollen of tropical trees (e.g., Celtis) were found in sites located at up to 25°N in southern Libya. In the Equatorial mountains, trees (e.g., Olea and Podocarpus) migrated to higher elevations to form the present-day Afro-montane forests. Patterns of migration were individualistic, with the entire range of some taxa displaced to higher latitudes or shifted from one elevation belt to another. New combinations of climate/environmental conditions allowed the cooccurrences of taxa growing today in separate regions. Such migrational processes and species-overlapping ranges led to a tremendous increase in biodiversity, particularly in the Saharan desert, where more humid-adapted taxa expanded along water courses, lakes, and wetlands, whereas xerophytic populations persisted in drier areas.
At the end of the Holocene era, some 2,500 to 4,500 years ago, the majority of sites in tropical Africa recorded a shift to drier conditions, with many lakes and wetlands drying out. The vegetation response to this shift was the overall disruption of the forests and the wide expansion of open landscapes (wooded grasslands, grasslands, and steppes). This environmental crisis created favorable conditions for further plant exploitation and cereal cultivation in the Congo Basin.