From Climate.gov:
This article is the second of two articles describing the hottest time periods in Earth’s history.
Throughout its 4.54-billion-year history, Earth has experienced multiple periods of temperatures hotter than today’s. But as far as the “recent” past, a study published in March 2013 concluded that global average temperature is now higher than it has been for most of the last 11,300 years.
The scientists assembled dozens of temperature records from multiple studies, including data from sediment cores drilled in lake bottoms and sea floors, and from ice cores. Assembling data from 73 records that overlap in time, the scientists pieced together global average temperatures since the end of the last ice age.
The 11,000-year temperature reconstruction shows global average temperature increasing after the end of the last ice age and leveling off about 7550 and 3550 BC. After that time, global temperatures dropped until the “Little Ice Age,” bottoming out somewhere between AD 1450 and 1850. Afterwards temperatures rose again, first slowly then very rapidly. (The estimated temperatures for the past 1,500 years correlated with previous research that covered the same time period.)
Natural variability can explain much of the temperature variation since the end of the last ice age, resulting from factors such as changes in the tilt of the Earth’s axis. Over the past century, though, global average temperatures have “risen from near the coldest to the warmest levels” in the past 11,300 years, the 2013 study authors explain. Over this same period, emissions of heat-trapping gases from human activities have increased.
Given the uncertainty inherent in estimating ancient temperatures, the scientists conservatively concluded that the last decade has brought global average temperatures higher than they have been for at least 75 percent of the last 11,300 years. The recent increase in global average temperature is so abrupt compared to the rest of the time period that when the scientists make a graph of the data, the end of the line is nearly vertical.
What about the future? To project future temperatures, the research team used greenhouse gas emission scenarios outlined in the Intergovernmental Panel on Climate Change (IPCC) Climate Change 2007: The Physical Science Basis report, and the authors expect the steep increase to continue through the year 2100 regardless of which one of the emission scenarios from the 2007 report is considered.
For most of the past 10,000 years, global average temperature has remained relatively stable and low compared to earlier hothouse conditions in our planet’s history. Now, temperature is among the highest experienced not only in the “recent” past—the past 11,000 years or so, during which modern human civilization developed—but also probably for a much longer period.
Carrie Morrill of the National Climatic Data Center explains, “You’d have to go back to the last interglacial [warm period between ice ages] about 125,000 years ago to find temperatures significantly higher than temperatures of today.”