Over the past 2 billion years, the Earth's climate
has repeatedly fluctuated
between average global temperatures
of approximately 12°C and 22°C. [These are measured on a geologic
timescale and are based on geologic evidence; obviously, there is some regional, annual and seasonal variation
.] When the Earth is close to 22°, it is said to be experiencing a "hothouse climate
," and arid
, and warm-temperate zones expand to cover most of the Earth
's surface. In an "icehouse climate"
, closer to 12°, the Earth's poles
are covered by ice caps
and significant polar and cool-temperate zones appear.
The trend (not always followed) has been long periods of hothouse followed by significantly shorter periods of icehouse. They are separated by "geologically quick" periods of transition. Approximately 50 million years ago, the Earth began cooling off at a signifcant rate from a long hothouse period. This cooling was especially rapid 30-35 million years ago, and was even more pronounced 10-15 million years ago. An icehouse climate came about, and polar ice returned to the Earth, though it probably came much earlier to the South Pole. [Earth has not always had this asymmetric climate, however.] Evidence suggests that the frigid peak of this icehouse climate was about 5-6 million years ago – last week in geologic terms. Since then, the Earth has been warming very slowly.
It should come as no surprise that we are still in an icehouse climate. Current average global temperature is 14° or 15°, depending on who you ask. We still have polar ice caps, snow, ice, 3°C ocean bottoms, and glaciation – all "abnormal" features of the Earth's surface. Over the past two million years there has been a great deal of variation in world temperature, but in the grand scheme of things these variations have been small and short-lived, even though they have been significant enough to substantially alter the course of human life! The recent "Ice Ages" (such as the one 18,000 years ago), serious as they are, are just noise in the overall graph of global temperature change. Even more geologically tiny have been changes since the last Ice Age, which in some cases are merely regional instead of global. Evidence suggests that Northern Europe was more than 1°C warmer than the present when the Vikings flourished. A few hundred years ago, Europe was going through the "Little Ice Age". Today, people are documenting the warming of the last 150 years and looking for a human connection.
None of these minor fluctuations change the big picture. Earth during the million-year human era is slightly warmer than the recent pre-human past, but much colder than the norm.