Natural variability can explain much of Earth's average temperature variation since the end of the last ice age, but over the past century, global average temperature has risen from near the coldest to the warmest levels in the past 11,300 years.
Temperature extremes have been pretty unusual across the United States so far in 2014. Looking back over this time period quickly reveals at least part of what was going on: the polar jet stream got into a serious rut.
On July 11, Lake Mead reached its lowest water level since construction of the Hoover Dam in the 1930s. Many scientists are concerned that the prolonged drought could be a sign that the region will face significant water supply challenges due to climate change.