Nearly a thousand years ago, there was a period we now call the medieval warm period. It spanned roughly 250 years from 900 to 1150 AD. It's so called because it featured a warmer climate than the dark ages before it or the cold little ice age that followed. But what caused this warmth? And how does it compare to today's temperatures? Averaged over the entire globe, temperatures during the medieval warm period were similar to the mid-20th century, according to records collected by scientists studying past climates. However, just like today, certain regions during this time warmed more than others. For example, the North Atlantic region warmed far more than the tropics. In areas where warming was the greatest, air temperatures were similar to the late 20th century, but are less than those seen over the last decade. The warmth of the medieval warm period made northern lands easier to access. In coastal Greenland, Vikings were able to build villages that survived for many decades before cooler conditions forced them to relocate. Although some areas became more livable, other areas became harsher. For instance, a thousand years ago, mega-droughts occurred in the southwestern United States, which made living the area much harder. The medieval warm period was caused by a combination of natural factors that led to a slow warming, which peaked around a thousand years ago. Likewise, the end of the medieval warm period was caused by these same factors acting to cool the climate and begin the little ice age. There are three main factors that scientists believe caused the warmth during the medieval warm period. First, scientists estimate that solar activity was greater during the medieval warm period than in the dark ages beforehand or the little ice age that followed. An increase in solar activity caused a warmer climate, especially in the summer. Second, during the medieval warm period, there were few volcanic eruptions, which inject dust into the atmosphere and reflect incoming radiation. The low volcanic activity during the medieval warm period allowed warm conditions to prevail for longer periods in between eruptions. There is also strong evidence that increasing volcanic activity was a major cause of cooling at the end of the medieval warm period. Third, the Earth's orbit was different a thousand years ago than it is today. Orbital changes influenced the amount of solar radiation received at the Earth's surface. According to the orbit today, the medieval warm period summers would have been warmer in the polar regions. Combining these three factors is enough to explain conditions during the medieval warm period. Studies with climate models have shown that air and ocean temperatures during this period can be reproduced by including these factors into climate models. Although we understand the main drivers of the climate of the medieval warm period, there is less certainty in other regards. For example, more knowledge is still needed to know how the regional recorded patterns developed and how the atmosphere and ocean reacted at that time. There are two common myths about the medieval warm period. The first myth argues that modern global warming could have been caused by the same natural factors that caused the medieval warm period. This is an example of jumping to a conclusion. The drivers of the medieval warm period are well understood and are not the same as those driving climate change today. If we only look at the natural factors of climate over the past 50 years, globally there should have been a cooling instead of warming. The only way to account for recent global warming is to include the increase of human emissions of greenhouse gases. Natural drivers can't have caused a recent warming. In fact, they have had a small cooling influence. The most common myth about the medieval warm period is that conditions during medieval times were warmer than now. However, when scientists reconstruct temperatures averaged over the whole planet, they find that the medieval warm period was not warmer than present. So how do people argue that the medieval warm period was warmer? They do this by cherry picking single locations and comparing medieval temperatures to today's temperatures at that location. One location may have in fact been warmer in medieval times and nearby location may have been cooler. We need to average out over huge areas to be able to compare medieval climate to today's climate. There is another important piece of data that also confirms that the medieval warm period was not as warm, and this is sea levels. If it were warmer during the medieval warm period, then sea levels would have been higher and would have risen faster than today. This is for two reasons. First, more ice would have melted and secondly, warming ocean water expands in volume. We see no evidence of sea level rise like today and reconstructions of sea level rise. This is strong evidence that the current warming exceeds that of the medieval warm period. The global temperatures being experienced now are unprecedented for at least the last 1,000 years.