late 14c., "horizontal zone of the earth's surface measured by lines parallel to the equator," from Old French climat "region, part of the earth," from Latin clima (genitive climatis) "region; slope of the earth," from Greek klima "region, zone," literally "an inclination, slope," thus "slope of the earth from equator to pole," from a suffixed form of PIE root *klei- "to lean."

Ancient geographers divided the earth into zones based on the angle of sun on the slope of the earth's surface and the length of daylight. Some reckoned 24 or 30 climates between Meroe on the upper Nile in Sudan and the mythical Riphaean Mountains which were supposed to bound the Arctic; a change of climate took place, going north, at a place where the day was a half hour longer or shorter, according to season, than the starting point. Others counted 7 (each dominated by a particular planet) or 12 (dominated by zodiac signs).

Change of temperature gradually came to be considered more important, and by late 14c. the word was being used in the sense "a distinct region of the earth's surface considered with respect to weather." The sense shift to "combined results of weather associated with a region, characteristic condition of a country or region with reference to the variation of heat, cold, rainfall, wind, etc.," is attested by c. 1600. Figuratively, of mental or moral atmosphere, from 1660s.

1983, in the modern "human-caused global warming" sense. See climate (n.) + change (n.). Climatic change in a similar sense was in use from 1975.

"scientific study of climates," 1803, from climate + -ology.  In 19c., more or less equivalent to meteorology. 

"climate of a very small or restricted area," 1918, from micro- + climate. Related: Microclimatology; microclimatological.

"a description or study of climates," 1813, from climate + -graphy, with connective -o-. Related: Climatographic; climatographical; climatographically.

"related to or connected with climate," 1803, from climate + -ic. There is a 1650 citation for climatical in OED. Climatal (1830) is marked "rare" in Century Dictionary. Related: Climatically.

1540s, "a tract or region of the earth," shortening of climate (or a nativization of Latin clima). It might usefully take up the old, abandoned "horizontal region of the earth" sense of climate, but it is used chiefly by the poets, and they display no evident agreement on what they mean by it.

1792, "habituate (something) to a new climate," from French acclimater, verb formed from à "to" (see ad-) + climat (see climate). Intransitive sense "adapt to a new climate" is from 1861. Related: Acclimated; acclimating. The extended form acclimatize is now more common in the older sense of this word (generally in reference to plants or animals), leaving to this word the intransitive sense, which more often refers to humans.

Proto-Indo-European root meaning "to lean."

It forms all or part of: acclivity; anticline; clemency; client; climate; climax; cline; clinic; clinical; clino-; clitellum; clitoris; decline; declivity; enclitic; heteroclite; incline; ladder; lean (v.); lid; low (n.2) "small hill, eminence;" matroclinous; patroclinous; polyclinic; proclitic; proclivity; recline; synclinal; thermocline.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian šlyti "to slope," šlieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting").

1824, "modify a living thing to suit a foreign climate" (transitive); see acclimate + -ize. A more recent formation than acclimate and generally replacing it in this sense. Related: Acclimatized; acclimatizing. Simple climatize is attested from 1826 as "inure (a living thing) to a climate."