Extreme heat in Mexico, Central America and parts of the US South has left millions sweltering, strained energy grids and resulted in iconic Howler monkeys in Mexico dropping dead from trees. Meteorologists say a large heat dome — an area of high pressure that blocks clouds from forming and results in extensive sunshine and hot temperatures — is the culprit. The heat dome is occurring in a world that's warming unnaturally fast due to human-caused emissions of greenhouse gases, especially carbon dioxide and methane.