72 Ecological Sustainability
was a so-called Medieval Warm Period from about 900 to 1200 AD. Fairly
normal temperature variations followed for about 200 years, and then there
was a “Little Ice Age” (LIA) from about 1400 to 1800 AD. After about 1900
AD, the mean planetary temperature has risen steadily.
The reason the Earth’s mean temperature is currently rising is that the
net planetary heat balance is positive; the Earth now is absorbing more heat
than it can radiate into space, hence the late 20th- and 21st-century average
temperature rise. The mean planetary temperature rises until at some higher
temperature, the blackbody (BB) radiation of long-wave infrared radiation
(LIR) energy into space from the planet’s surface balances the ...