lineage of beetles whose genetic makeup has not altered significantly over the millennia, knowledge of the present climatic range of the different species, and the age of the sediments in which remains are found, past climatic conditions may be inferred. analysis and uncertainties one difficulty in detecting climate cycles is that the earth's climate has been changing in non-cyclic ways over most paleoclimatological timescales. currently we are in a period of anthropogenic global warming. in a larger timeframe, the earth is emerging from the latest ice age, cooling from the holocene climatic optimum and warming from the "little ice age", which means that climate has been constantly changing over the last 15,000 years or so. during warm periods, temperature fluctuations are often of a lesser amplitude. the pleistocene period, dominated by repeated glaciations, developed out of more stable conditions in the miocene and pliocene climate. holocene climate has been relatively stable. all of these changes complicate the task of looking for cyclical behavior in the climate. positive feedback, negative feedback, and ecological inertia from the land-ocean-atmosphere system often attenuate or reverse smaller effects, whether from orbital forcings, solar variations or changes in concentrations of greenhouse gases. certain feedbacks involving processes such as clouds are also uncertain; for contrails, natural cirrus clouds, oceanic dimethyl sulfide and a land-based equivalent, competing theories exist concerning effects on climatic temperatures, for example contrasting the iris hypothesis and claw hypothesis. impacts life top: arid ice age climatemiddle: atlantic period, warm and wetbottom: potential vegetation in climate now if not for human effects like agriculture. vegetation a change in the type, distribution and coverage of vegetation may occur given a change in the climate. some changes in climate may result in increased precipitation and warmth, resulting in improved plant growth and the subsequent sequestration of airborne co2. the effects are expected to affect the rate of many natural cycles like plant litter decomposition rates. a gradual increase in warmth in a region will lead to earlier flowering and fruiting times, driving a change in the timing of life cycles of dependent organisms. conversely, cold will cause plant bio-cycles to lag. larger, faster or more radical changes, however, may result in vegetation stress, rapid plant loss and desertification in certain circumstances. an example of this occurred during the carboniferous rainforest collapse (crc), an extinction event 300 million years ago. at this time vast rainforests covered the equatorial region of europe and america. climate change devastated these tropical rainforests, abruptly fragmenting the habitat into isolated 'islands' and causing the extinction of many plant and animal species. wildlife one of the most important ways animals can deal with climatic change is migration to warmer or colder regions. on a longer timescale, evolution makes ecosystems including animals better adapted to a new climate. rapid or large climate change can cause mass extinctions when creatures are stretched too far to be able to adapt. humanity collapses of past civilizations such as the maya may be related to cycles of precipitation, especially drought, that in this example also correlates to the western hemisphere warm pool. around 70 000 years ago the toba supervolcano eruption created an especially cold period during the ice age, leading to a possible genetic bottleneck in human populations. glaciers and ice sheets glaciers are considered among the most sensitive indicators of a changing climate. their size is determined by a mass balance between snow input and melt output. as temperatures increase, glaciers retreat unless snow precipitation increases to make up for the additional melt. glaciers grow and shrink due both to natural variability and external forcings. variability in temperature, precipitation and hydrology can strongly determine the evolution of a glacier in a particular season. the most significant climate processes since the middle to late pliocene (approximately 3 million years ago) are the glacial and interglacial cycles. the present interglacial period (the holocene) has lasted about 11,700 years. shaped by orbital variations, responses such as the rise and fall of continental ice sheets and significant sea-level changes helped create the climate. other changes, including heinrich events, dansgaard–oeschger events and the younger dryas, however, illustrate how glacial variations may also influence climate without the orbital forcing. sea level change during the last glacial maximum, some 25,000 years ago, sea levels were roughly 130 m lower than today. the deglaciation afterwards was characterized by rapid sea level change. in the early pliocene, global temperatures were 1–2˚c warmer than the present temperature, yet sea level was 15–25 meters higher than today. sea ice sea ice plays an important role in earth's climate as it affects the total amount of sunlight that is reflected away from the earth. in the past, the earth's oceans have been almost entirely covered by sea ice on a number of occasions, when the earth was in a so-called snowball earth state, and completely ice-free in periods of warm climate. when there is a lot of sea ice present globally, especially in the tropics and subtropics, the climate is more sensitive to forcings as the ice–albedo feedback is very strong. climate history various climate forcings are typically in flux throughout geologic time, and some processes of the earth's temperature may be self-regulating. for example, during the snowball earth period, large glacial ice sheets spanned to earth's equator, covering nearly its entire surface, and very high albedo created extremely low temperatures, while the accumulation of snow and ice likely removed carbon dioxide through atmospheric deposition. however, the absence of plant cover to absorb atmospheric co2 emitted by volcanoes meant that the greenhouse gas could accumulate in the atmosphere. there was also an absence of exposed silicate rocks, which use co2 when they undergo weathering. this created a warming that later melted the ice and brought earth's temperature back up. paleo-eocene thermal maximum climate changes over the past 65 million years, using proxy data including oxygen-18 ratios from foraminifera. the paleocene–eocene thermal maximum (petm) was a time period with more than 5–8 °c global average temperature rise across the event. this climate event occurred at the time boundary of the paleocene and eocene geological epochs. during the event large amounts of methane was released, a potent greenhouse gas. the petm represents a "case study" for modern climate change as in the greenhouse gases were released in a geologically relatively short amount of time. during the petm, a mass extinction of organisms in the deep ocean took place. the cenozoic throughout the cenozoic, multiple climate forcings led to warming and cooling of the atmosphere, which led to the early formation of the antarctic ice sheet, subsequent melting, and its later reglaciation. the temperature changes occurred somewhat suddenly, at carbon dioxide concentrations of about 600–760 ppm and temperatures approximately 4 °c warmer than today. during the pleistocene, cycles of glaciations and interglacials occurred on cycles of roughly 100,000 years, but may stay longer within an interglacial when orbital eccentricity approaches zero, as during the current interglacial. previous interglacials such as the eemian phase created temperatures higher than today, higher sea levels, and some partial melting of the west antarctic ice sheet. climatological temperatures substantially affect cloud cover and precipitation. at lower temperatures, air can hold less water vapour, which can lead to decreased precipitation. during the last glacial maximum of 18,000 years ago, thermal-driven evaporation from the oceans onto continental landmasses was low, causing large areas of extreme desert, including polar deserts (cold but with low rates of cloud cover and precipitation). in contrast, the world's climate was cloudier and wetter than today near the start of the warm atlantic period of 8000 years ago. the holocene temperature change over the past 12 000 years, from various sources. the thick black curve is an average. the holocene is characterized by a long-term cooling starting after the holocene optimum, when temperatures were probably only just below current temperatures (second decade of the 21st century), and a strong african monsoon created grassland conditions in the sahara during the neolithic subpluvial. since that time, several cooling events have occurred, including: the piora oscillation the middle bronze age cold epoch the iron age cold epoch the little ice age the phase of cooling c. 1940–1970, which led to global cooling hypothesis in contrast, several warm periods have also taken place, and they include but are not limited to: a warm period during the apex of the minoan civilization the roman warm period the medieval warm period modern warming during the 20th century certain effects have occurred during these cycles. for example, during the medieval warm period, the american midwest was in drought, including the sand hills of nebraska which were active sand dunes. the black death plague of yersinia pestis also occurred during medieval temperature fluctuations, and may be related to changing climates. solar activity may have contributed to part of the modern warming that peaked in the 1930s. however, solar cycles fail to account for warming observed since the 1980s to the present day . events such as the opening of the northwest passage and recent record low ice minima of the modern arctic shrinkage have not taken place for at least several centuries, as early explorers were all unable to make an arctic crossing, even in summer. shifts in biomes and habitat ranges are also unprecedented, occurring at rates that do not coincide with known climate oscillations . modern climate change and global warming main article: climate change as a consequence of humans emitting greenhouse gases, global surface temperatures have started rising. global warming is an aspect of modern climate change, a term that also includes the observed changes in precipitation, storm tracks and cloudiness. as a consequence, glaciers worldwide have been found to be shrinking significantly. land ice sheets in both antarctica and greenland have been losing mass since 2002 and have seen an acceleration of ice mass loss since 2009. global sea levels have been rising as a consequence of thermal expansion and ice melt. the decline in arctic sea ice, both in extent and thickness, over the last several decades is further evidence for rapid climate change. variability between regions examples of regional climate variability land-ocean. surface air temperatures over land masses have been increasing faster than those over the ocean, the ocean absorbing about 90% of excess heat. hemispheres. the hemispheres' average temperature changes have diverged because of the north's greater percentage of landmass, and due to global ocean currents. latitude bands. three latitude bands that respectively cover 30, 40 and 30 percent of the global surface area show mutually distinct temperature growth patterns in recent decades. altitude. a warming stripes graphic (blues denote cool, reds denote warm) shows how the greenhouse effect traps heat in the lower atmosphere so that the upper atmosphere, receiving less reflected energy, cools. volcanos cause upper-atmosphere temperature spikes. global versus regional. for geographical and statistical reasons, larger year-to-year variations are expected for localized geographic regions (e.g., the caribbean) than for global averages. relative deviation. though northern america has warmed more than its tropics, the tropics have more clearly departed from normal historical variability (colored bands: 1σ, 2σ standard deviations). in addition to global climate variability and global climate change over time, numerous climatic variations occur contemporaneously across different physical regions. the oceans' absorption of about 90% of excess heat has helped to cause land surface temperatures to grow more rapidly than sea surface temperatures. the northern hemisphere, having a larger landmass-to-ocean ratio than the southern hemisphere, shows greater average temperature increases. variations across different latitude bands also reflect this divergence in average temperature increase, with the temperature increase of northern extratropics exceeding that of the tropics, which in turn exceeds that of the southern extratropics. upper regions of the atmosphere have been cooling contemporaneously with a warming in the lower atmosphere, confirming the action of the greenhouse effect and ozone depletion. observed regional climatic variations confirm predictions concerning ongoing changes, for example, by contrasting (smoother) year-to-year global variations with (more volatile) year-to-year variations in localized regions. conversely, comparing different regions' warming patterns to their respective historical variabilities, allows the raw magnitudes of temperature changes to be placed in the perspective of what is normal variability for each region. regional variability observations permit study of regionalized climate tipping points such as rainforest loss, ice sheet and sea ice melt, and permafrost thawing. such