Temperatures over the past 10years clearly were the warmest since measured records have been available (Stefan et al., 2003). The year 1998 sticks out well above the long-term trend due to the occurrence of major El Nino events that year (the last El Nino so far and one of the strongest on records).These events are examples of the largest natural climate variations on multiple year time scales and by releasing heat from the oceans generally cause positive anomalies in global mean temperature (Stefan et al., 2003. It is remarkable that the year 2005 rivaled the heat of 1998 even though no El Nino event occurred that year. A bizarre curiosity, perhaps worth mentioning is that several prominent climatic skeptics recently used the extreme year 1998 to claim in the media that global warming has ended as glaciers respond sensitively to temperature changes. Data on extend of glaciers have been used to reconstruct a history of Northern hemisphere temperature over the past four centuries.
The following evidences of climate change are explained below.
I.Instrumental climate data:-Instrumental records are by far the most reliable, all available climate data-they are precisely dated ,require either to no explicit calibration, or employed physically based calibration(E.g a mercury thermometer) These data which include thermometer-based surface temperature measurement from the ocean and land regions ,sea level pressure (SLP)measurements, continental and oceanic precipitation measurement (including drought indices). Sea ice extents and winds and humidity estimates are however only available on a widespread bases back 1850(Jones et al., 1999)
II. Tree Ring Records:-Tree ring or dendro-climatic proxy climate indicators can provide information regarding past seasonal temperature or precipitation/drought, based on measurements of annual ring widths and/or maximum late wood densities (Fritts et al.,1991;Briffa,1992; Osborn,2002) carbon and oxygen isotopes from tree rings have been considered as potential climatic proxies ,but a straight forward interpretation of the measurements has proven elusive ,while there has been some promising recent work in this area9leavitt et al.,1995;r Robertson et al.,1997;Anderson et al .,1998). Tree rings Isotopes have not been used in large scale climate reconstruction.
III. Ice Core Records:- Ice core provides climate information over multiple millennia from the polar region of both northern hemisphere(OBrien et al.,1995;Fisheret al 1997;Vinther et al.,2003).as well as alpine (tropical and sub-tropical) environment (Thomson et al.,2003).Ice core can provide several climatic related indicators, including the fraction of melting ice, the rate of accumulation of and concentration precipitation of various chemical constituents ( including trace gases).that provides information about the atmospheric environment( OBrien et al.,1995;Meeker and Mayewski.,2002).
IV. Speleothems:- Speleothems (cave deposits) are formed as part of the meteoric water cycle and variations in their growth rates and composition reflect environmental changes on the land surface above the cave (Lauritzen and Lundberg.,1999 ). Various proxies had been studied; including growth rates Isotopic-composition trace elements, organic matter content and luminescent laminae.
V. Varved Lake and Ocean Sediment Records:-Annually laminated (varved lake sediments like ice cores, provides high-resolution proxy climate information in high latitude regions where other proxy indicators, such as tree rings are not available (Lamoureux and Bradley,1996;Hughenet coal.,2001). Varved sediments formed from the deposition of inorganic sediments (clastic ) are controlled by seasonal precipitation and summer temperature, both of which govern the volume of melt water discharge and sediments load into a closed basin glacial lake. When the latter process dominates the deposition process, the annual time and (or even sub-annual) varved thickness provides inferences into summer temperature variation (Over peck et al., 1997; Hughen et al., 2001).
VI. Bore Holes: – Bore holes can provide estimates of past temperature variability, through their resolution. It is intrinsically multi-decadal at its finest and decreases back in time. Bore hole derived temperature estimates from two distinct sources: wide spread terrestrial bore-holes in tropical, mid-latitude and sub polar environment (Beltram et al.,1995;Hrris and Chapman,1997,2001;and Mann et al.,2003).and ice bore holes from polar ice caps(Dahl-Jensen.,1998). In both cases, the sub surface temperature profiles are used to obtain an estimate of ground (or ice) surface temperature (GST) changes back in time through solution of an appropriate inverse problem. See figure.
VII. Glacier evidence:-the position of glacial moraines (till left behind receding glaciers) can provide information on the advances (and less accurately the retreats) of mountain glaciers in past centuries. Simple interpretation of such information in terms of past surface air temperature and precipitation changes is however confounded by the delicate balance between local changes in melting and ice accumulation and the variable response time scale of glacial mass balance ( which typically increases with the size of the glacier, potentially approaching century time scale for large mountain glacier).(Broeker.,2001).