Declining Humidity Is Defying Global Warming Models

Published September 10, 2014

Atmospheric relative humidity has substantially declined in recent decades, defying global warming computer models predicting higher amounts of atmospheric water vapor that will exacerbate global warming. The decline in relative humidity indicates global warming will be much more moderate than global warming activists claim.

CO2 Has Minimal Impact

Carbon dioxide’s impact on global temperatures is not in dispute. As a matter of physics, doubling atmospheric water vapor from pre-Industrial Age levels will directly cause approximately 1 degree Celsius of warming. From the dawn of the Industrial Revolution until today, atmospheric carbon dioxide has risen by merely 40 percent. Accordingly, carbon dioxide has directly caused approximately 0.4 degrees Celsius of warming (actually a little more, as the earlier increases in atmospheric carbon dioxide will trap more long wave radiation than later increases). If global carbon dioxide emissions continue at their current pace, we can expect human-caused carbon dioxide emissions will directly cause at most another 0.6 degrees Celsius this century.

Humidity Predictions More Important

United Nations computer models, however, predict approximately 2.4 degrees Celsius of 21st century warming. The discrepancy arises because the computer models are programmed to assume that whenever temperatures warm—due to increasing carbon dioxide emissions or other reasons—a small amount of initial warming creates a cascade effect of other factors that induce even more warming.

The most important of these assumptions is that a little bit of carbon dioxide-induced warming will create a substantial increase in atmospheric water vapor. Water vapor is a much more potent greenhouse gas than carbon dioxide, so substantial increases in atmospheric water vapor can certainly cause significant warming. United Nations computer models are programmed to assume absolute humidity (the total amount of water vapor in the atmosphere) will rise so much that even relative humidity (the percent of water vapor in the atmosphere) will at least keep pace and perhaps even increase. Warmer air is able to hold more water than cooler air, so absolute water vapor would have to increase quite substantially for relative humidity to remain constant or increase in a warming world.

Relative Humidity Is Declining

Scientists, however, have been measuring relative humidity for many decades. Rather than keeping pace with modestly warming temperatures, relative humidity is declining. This decline has been ongoing, without interruption, for more than 60 years. After more than six decades of consistent data, we can say with strong confidence that absolute humidity is not rising rapidly enough for relative humidity to keep pace with warming temperatures.

The failure of relative humidity to hold constant or rise during recent decades is a lethal dagger in the heart of alarmist global warming claims. According to the UN computer models, rising humidity will cause substantially more global warming than the modest warming directly caused by rising carbon dioxide levels. Given the potency of water vapor, even a small overstatement of atmospheric humidity levels in UN computer models will cause a very significant overstatement of future warming. And the data show UN computer models assume too much atmospheric humidity.

Models’ Predictions Were Wrong

The effects of this overstatement are apparent in real-world temperature data this century. Precise atmospheric temperature measurements compiled by NASA and NOAA satellite instruments show there has been no global warming since late in the 20th century. Some global warming activists claim some of the data indicate there may still have been a small amount of warming in recent years, but even a minor warming contradicts UN computer models claiming we should be experiencing rapid warming. If the Earth were truly going to warm 2.4 degrees Celsius this century, we should have already experienced approximately 0.35 degrees Celsius warming. The difference between no warming and 0.35 degrees Celsius warming may not sound like much of a discrepancy at first blush, but the Earth only warmed approximately 0.60 degrees Celsius during the entirety of the 20th century. As United Nations Intergovernmental Panel on Climate Change (IPCC) lead author Hans von Storch observed in June 2013 in der Spiegel, “If things continue as they have been, in five years, at the latest, we will need to acknowledge that something is fundamentally wrong with our climate models. A 20-year pause in global warming does not occur in a single modeled scenario. But even today, we are finding it very difficult to reconcile actual temperature trends with our expectations.”

The UN can go a long way toward correcting its climate models if by simply admitting it has overstated the impact of modest carbon dioxide-created global warming on atmospheric humidity. More accurate climate models will allow for a better informed discussion on global warming, and will go a long way toward bridging the divide in an increasingly volatile scientific and political debate.

James M. Taylor ([email protected]) is senior fellow for environment policy at The Heartland Institute. This article first appeared at, reprinted with permission.