The Pliocene is the geological period from about 5 million years ago to about 2.5 million years ago. The climate then:
The global average temperature in the mid-Pliocene (3.3 Ma–3 Ma) was 2–3 °C higher than today, global sea level 25m higher and Northern hemisphere ice sheet ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma.and
Carbon dioxide concentration during the mid Pliocene has been estimated at around 400 ppmv from 13C/12C ratio in organic marine matter and stomatal density of fossilized leaves, decreasing carbon dioxide levels during late Pliocene may have contributed substantially to global cooling and the onset of northern hemisphere glaciation.So the Pliocene is often thought of as a potential analog for the future anthropocene climate (given that we'll be hitting 400ppm in about 2015 on the current trajectory). It's the last geological era before the great ice ages and thus the last time the planet was as warm as humanity is about to make it.
However, the Pliocene was very likely wetter than the modern climate on average. My understanding is that the evidence for this comes primarily from studies of ancient vegetation - chiefly by classifying pollen in sediment cores in lakes or under the ocean. A nice synthesis of this seems to be the PRISM3 project which has produced a dataset of vegetation types around the globe. From this data I produced these maps which show the Pliocene vegetation on top and modern vegetation on the bottom:
Click for a big version in a new window. Here the key is as follows: yellow is desert, dark green is various forms of forest, pale green is various forms of grassland, scrub, or savannah, and grey is ice or tundra. There are three things that seem to be most striking about the differences:
- There was far less desert in the Pliocene with most of the current deserts being grassland or savannahs.
- There was more tropical forest in the Pliocene.
- Temperate and boreal forests were generally pushed north with more grassland/savannah in currently temperate locations and forests where the tundra/ice is now.
It's the first two points that seem key to the idea that the Pliocene was wetter - there's no evidence of the kind of pattern of overall drying that we seem to be seeing in recent decades: