Global lower stratospheric temperatures in January to March 2012 were the coldest ever measured (by satellites) according to Remote Sensing Systems (RSS). A cooling lower stratosphere is strong evidence of a strengthening greenhouse effect caused by increasing levels of greenhouse gases in the lower atmosphere (the troposphere). Heat from the lower atmosphere is radiated upwards at lower temperatures as increasing levels of greenhouse gases trap more heat in the troposphere. The lower stratosphere, which lies just above the troposphere receives less heat from below. Lower stratospheric cooling was predicted by climate scientists before it was observed. The combination of a warming troposphere and cooling lower stratosphere cannot be explained by external factors such as a warming sun or more cosmic rays. A warming sun would warm both the troposphere and the lower stratosphere.
Two years of La Nina, an event where trade winds and upwelling of cold equatorial Pacific water increase, have cooled equatorial ocean sea surface temperatures. Global surface temperatures have cooled in response to La Nina over the past 2 years.
However, globally, March was the coolest since 1999. La Nina's strong trade winds moved greater than normal amounts of oceanic heat from the equatorial oceans to the sub tropics and mid latitudes. The northern hemisphere jet stream was displaced northwards over the Pacific, North America and the Atlantic in response to the heat transfer in the oceans.
Gulf of Mexico temperatures were the warmest ever measured in March according to Jeff Masters. The record warm Gulf was the source of air, with higher than normal amounts of precipitable water, that fueled March's tornado outbreaks.
The United States had record March heat while the rest of the world was relatively cool because of the unprecedented ocean-atmosphere circulation patterns.
The cooling of La Nina and warming of El Nino add complexity to the surface temperature record. This complexity contributes to weather variability, making it hard to perceive the warming climate over the short term record. Skeptics can cherry pick short term records to say "Global warming has stopped".
However, the combination of a warming lower atmosphere and a cooling stratosphere is undeniable evidence of an intensifying greenhouse effect that is caused by increasing levels of man-made greenhouse gases in the atmosphere.