Query: A drop of volcanic activity in the early 20th century may have had a warming effect.

Relevant passages:
African humid period <=SEP=> The African humid period (AHP) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, trees and lakes was caused by changes in Earth's orbit around the Sun; changes in vegetation and dust in the Sahara which strengthened the African monsoon; and increased greenhouse gases, which may imply that anthropogenic global warming could result in a shrinkage of the Sahara desert. During the preceding last glacial maximum, the Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, but its lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of Heinrich event 1, simultaneously to the Bølling-Allerød warming. Rivers and lakes such as Lake Chad formed or expanded, glaciers grew on Mount Kilimanjaro and the Sahara retreated. Two major dry fluctuations occurred; during the Younger Dryas and the short 8.2 kiloyear event. The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the Sahel, Arabia and East Africa the period appears to have taken place in several steps such as the 4.2 kiloyear event. The AHP led to a widespread settlement of the Sahara and the Arabian Deserts, and had a profound effect on African cultures, such as the birth of the Pharaonic civilization. They lived as hunter-gatherers until the agricultural revolution and domesticated cattle, goats and sheep. They left archeological sites and artifacts such as one of the oldest ships in the world, and rock paintings such as those in the Cave of Swimmers and in the Acacus Mountains. Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. When the period ended, humans gradually abandoned the desert in favour of regions with more secure water supplies, such as the Nile Valley and Mesopotamia, where they gave rise to early complex societies.

Climate change (general concept) <=SEP=> Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. In the time since the industrial revolution the climate has increasingly been affected by human activities that are causing global warming and climate change.The climate system receives nearly all of its energy from the sun. The climate system also radiates energy to outer space. The balance of incoming and outgoing energy, and the passage of the energy through the climate system, determines Earth\'s energy budget. When the incoming energy is greater than the outgoing energy, earth\'s energy budget is positive and the climate system is warming. If more energy goes out, the energy budget is negative and earth experiences cooling.\nThe energy moving through Earth\'s climate system finds expression in weather, varying on geographic scales and time. Long-term averages and variability of weather in a region constitute the region\'s climate. Such changes can be the result of "internal variability", when natural processes inherent to the various parts of the climate system alter the distribution of energy. Examples include variability in ocean basins such as the Pacific decadal oscillation and Atlantic multidecadal oscillation. Climate variability can also result from external forcing, when events outside of the climate system\'s components nonetheless produce changes within the system. Examples include changes in solar output and volcanism.\nClimate variability has consequences for sea level changes, plant life, and mass extinctions; it also affects human societies.

Lake Tahoe <=SEP=> Lake Tahoe ( -LSB- ˈtɑːhoʊ -RSB- ) is a large freshwater lake in the Sierra Nevada of the United States . Lying at 6225 ft , it straddles the border between California and Nevada , west of Carson City . Lake Tahoe is the largest alpine lake in North America , and at 122160280 acre.ft trails only the five Great Lakes as the largest by volume in the United States . Its depth is 1645 ft , making it the second deepest in the United States after Crater Lake in Oregon ( 1945 ft ) .   The lake was formed about 2 million years ago as part of the Lake Tahoe Basin , with the modern extent being shaped during the ice ages . It is known for the clarity of its water and the panorama of surrounding mountains on all sides . The area surrounding the lake is also referred to as Lake Tahoe , or simply Tahoe . More than 75 % of the lake 's watershed is national forest land , comprising the Lake Tahoe Basin Management Unit of the United States Forest Service .   Lake Tahoe is a major tourist attraction in both Nevada and California . It is home to winter sports , summer outdoor recreation , and scenery enjoyed throughout the year . Snow and ski resorts are a significant part of the area 's economy and reputation . The Nevada side also offers large casinos , with highways providing year-round access to the entire area .

1257 Samalas eruption <=SEP=> The 1257 Samalas eruption was a major volcanic eruption of the Samalas volcano , next to Mount Rinjani on the island of Lombok , Indonesia . It left a large caldera next to Rinjani volcano , the lake Segara Anak . This volcanic eruption reached a volcanic explosivity index of 7 , making it one of the largest eruptions of the Holocene . The occurrence of this eruption was originally deduced from ice core information revealing a spike in sulfate deposition around the year 1257 ; in 2013 the source of this eruption at Samalas was definitively proven as a result of historical records from that time .   This eruption proceeded during four distinct phases , alternately creating eruption columns reaching up for tens of kilometres and pyroclastic flows that buried large parts of Lombok , destroying human habitations including the city of Pamatan . Ash from the eruption fell as far as Java . The volume of the deposits left exceeds 10 km3 . Later activity from the volcano created additional volcanic centres in the caldera , including the present day active Barujari cone .   The eruption was witnessed by humans on Lombok , who recorded its effects on text written on palm leaves . The aerosols injected by the volcano into the atmosphere reduced the solar radiation reaching the surface of the Earth , triggering a volcanic winter and cooling lasting for several years . It is possible that the 1257 Samalas eruption helped trigger the Little Ice Age .

Cloud <=SEP=> In meteorology , a cloud is an aerosol comprising a visible mass of minute liquid droplets , frozen crystals , or particles suspended in the atmosphere above the surface of a planetary body . The droplets and crystals may be made of water or various chemicals . On Earth , clouds are formed as a result of saturation of the air when it is cooled to its dew point , or when it gains sufficient moisture ( usually in the form of water vapor ) from an adjacent source to raise the dew point to the ambient temperature . They are seen in the Earth 's homosphere ( which includes the troposphere , stratosphere , and mesosphere ) . Nephology is the science of clouds which is undertaken in the cloud physics branch of meteorology .   There are two systems of naming clouds in their respective layers of the atmosphere ; Latin in the troposphere and mostly alpha-numeric above the troposphere . Cloud types in the troposphere , the atmospheric layer closest to Earth 's surface , have Latin names due to the universal adaptation of Luke Howard 's nomenclature . Formally proposed in 1802 , it became the basis of a modern international system that classifies clouds into five physical forms and three altitude levels ( formerly known as étages ) . These physical types , in approximate ascending order of convective activity , include stratiform sheets , cirriform wisps and patches , stratocumuliform layers ( mainly structured as rolls , ripples , and patches ) , cumuliform heaps , and very large cumulonimbiform heaps that often show complex structure . The physical forms are cross-classified by the altitude levels to produce ten basic genus-types , most of which can be divided into species , and subdivided into varieties .   Two cirriform clouds that form higher up in the stratosphere and mesosphere have common names for their main types , but are sub-classified alpha-numerically . They are relatively uncommon and are mostly seen in the polar regions of Earth . Clouds have been observed in the atmospheres of other planets and moons in the Solar System and beyond . However , due to their different temperature characteristics , they are often composed of other substances such as methane , ammonia , and sulfuric acid as well as water .   Homospheric types determined by cross-classification of forms and levels .   '' Homospheric types include the ten tropospheric genera and two additional major types above the troposphere . The cumulus genus includes three variants as defined by vertical size .