Query: The long term trend from albedo is of cooling.

Relevant passages:
Little Ice Age <=SEP=> The Little Ice Age ( LIA ) was a period of cooling that occurred after the Medieval Warm Period . Although it was not a true ice age , the term was introduced into scientific literature by François E. Matthes in 1939 . It has been conventionally defined as a period extending from the 16th to the 19th centuries , but some experts prefer an alternative timespan from about 1300 to about 1850 . Climatologists and historians working with local records no longer expect to agree on either the start or end dates of the period , which varied according to local conditions .   The NASA Earth Observatory notes three particularly cold intervals : one beginning about 1650 , another about 1770 , and the last in 1850 , all separated by intervals of slight warming . The Intergovernmental Panel on Climate Change Third Assessment Report considered the timing and areas affected by the Little Ice Age suggested largely-independent regional climate changes rather than a globally-synchronous increased glaciation . At most , there was modest cooling of the Northern Hemisphere during the period .   Several causes have been proposed : cyclical lows in solar radiation , heightened volcanic activity , changes in the ocean circulation , variations in Earth 's orbit and axial tilt ( orbital forcing ) , inherent variability in global climate , and decreases in the human population .

Ice age <=SEP=> An ice age is a period of long-term reduction in the temperature of Earth 's surface and atmosphere , resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers . Within a long-term ice age , individual pulses of cold climate are termed `` glacial periods '' ( or alternatively `` glacials '' or `` glaciations '' or colloquially as `` ice age '' ) , and intermittent warm periods are called `` interglacials '' . In the terminology of glaciology , ice age implies the presence of extensive ice sheets in both northern and southern hemispheres . By this definition , we are in an interglacial period -- the Holocene -- of the ice age . The ice age began 2.6 million years ago at the start of the Pleistocene epoch , because the Greenland , Arctic , and Antarctic ice sheets still exist .

Albedo <=SEP=> Albedo ( -LSB- ælˈbiːdoʊ -RSB- ) is a measure for reflectance or optical brightness ( Latin albedo , Arabic albayad , `` whiteness '' ) . It is dimensionless and measured on a scale from zero ( corresponding to a black body that absorbs all incident radiation ) to one ( corresponding to a white body that reflects all incident radiation ) .   Surface albedo is defined as the ratio of radiation reflected to the radiation incident on a surface . The proportion reflected is not only determined by properties of the surface itself , but also by the spectral and angular distribution of solar radiation reaching the Earth 's surface . These factors vary with atmospheric composition , geographic location and time ( see Position of the Sun ) . While bi-hemispherical reflectance is calculated for a single angle of incidence ( i.e. , for a given position of the sun ) , albedo is the directional integration of reflectance over all solar angles in a given period . The temporal resolution may range from seconds ( as obtained from flux measurements ) to daily , seasonal or annual averages .   Unless given for a specific wavelength ( spectral albedo ) , albedo refers to the entire spectrum of solar radiation . Due to measurement constraints , it is often given for the spectrum in which most solar energy reaches the surface ( approximately between 0.3 and 3 μm ) . This spectrum includes visible light ( 0.39-0 .7 μm ) , which explains why surfaces with a low albedo appear dark ( e.g. , trees absorb most radiation ) , whereas surfaces with a high albedo appear bright ( e.g. , snow reflects most radiation ) .   Albedo is an important concept in climatology , astronomy , and environmental management ( e.g. , as part of the Leadership in Energy and Environmental Design ( LEED ) program for sustainable rating of buildings ) . The average albedo of the Earth at the top of the atmosphere , its planetary albedo , is 30 to 35 % because of cloud cover , but widely varies locally across the surface because of different geological and environmental features .   The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria .