A new report in Science underscores what many scientists have been saying for years, it's water vapor, not CO₂, that has been driving global temperature changes in recent decades. Stratospheric water vapor concentrations decreased by about 10% after the year 2000, slowing the rate of global surface temperature increase over the past 10 years. It also seems likely that water vapor in the stratosphere increased between 1980 and 2000, causing surface temperatures to warm by an extra 30% during the 1990s. These findings show that stratospheric water vapor represents an important driver of decadal global surface climate change, yet the IPCC crowd continues to focus on CO₂.

The new report, “Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming,” by Susan Solomon et al. states that from 2000 to 2009 diminished water vapor levels in the upper atmosphere depressed global warming by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% compared to estimates neglecting this change.

The water vapor content of the atmosphere is highly variable, ranging from ~0 to 4%. Approximately 99% is contained in the troposphere but it is also present at higher altitudes. Increased stratospheric water vapor acts to cool the stratosphere but it warms the underlying troposphere. Unsurprisingly, the reverse is true for stratospheric water vapor decreases, the stratosphere warms but temperatures near the Earth's surface cool. Previous studies have suggested that stratospheric water vapor may contribute significantly to climate change, the question is by how much. Limited data are available prior to the mid-1990s, making the identification of systematic changes in atmospheric water vapor difficult. Because of calibration issues and limited spatial coverage the magnitude of the radiative effects are also hard to quantify.

Global mean atmospheric water vapor. Image NASA.

Water vapor gets into the atmosphere from a number of sources. For instance, water vapor is consistently the most common volcanic gas, accounting for more than 60% of total emissions during a surface eruption. But it is water evaporating from the surface of the oceans that provides most of the water vapor in Earth's atmosphere. Tropospheric water vapor increases in close association with warming and this represents a major climate feedback. The condensation of water vapor into liquid or ice creates for clouds, rain, snow, and other forms of precipitation.

Most of the phenomena that we experience as weather is caused by water vapor. Less obviously, the latent heat of vaporization is one of the most important terms in the atmospheric energy budget on both local and global scales. The heat energy absorbed by liquid H₂O as it turns into water vapor is later released into the atmosphere whenever condensation occurs. For example, latent heat release in atmospheric convection is directly responsible for powering destructive storms such as tropical cyclones and severe thunderstorms.

The feedback loop caused by evaporation is simulated in global climate models. In sharp contrast, current global models are limited in their representations of key processes that control the distribution and variability of water within the stratosphere. There is deep convection that affects the temperatures at which air enters the stratosphere, which results in drying. According to the research article: “Current global climate models simulate lower stratospheric temperature trends poorly and even up-to-date stratospheric chemistry-climate models do not consistently reproduce tropical tropopause minimum temperatures or recently observed changes in stratospheric water vapor.”

Layers of the atmosphere. Image UCAR.

The tropopause is the boundary between the troposphere, the lowest portion of the atmosphere, and the stratosphere, the second major atmospheric layer. Going upward from the surface, it is the point where air ceases to cool with height. More formally, it is the region of the atmosphere where the lapse rate—the rate at which temperature decreases with height—changes from positive to negative. In the stratosphere the warmer layers are higher up and cooler layers farther down. This is the reverse of the troposphere, which is cooler higher up and warmer farther down. How water vapor gets from the lower atmosphere into the stratosphere has been poorly understood. The Science article goes on to state that, in the real world, the contributions of changes in stratospheric water vapor to global climate change may be a source of unforced decadal variability, or they may be a feedback coupled to climate change.

The research assumed that between 1980 and the 1996–2000 period water vapor had increased uniformly by 1 ppmv at all latitudes and altitudes above the tropopause. A total globally averaged radiative forcing of +0.24 W m–2 was obtained for this assumed 1 ppmv increase. By comparison, the radiative forcing increase due to the growth of carbon dioxide was estimated at about +0.36 W m–2 from 1980–1996. “The comparison of these radiative forcings,” the author's state, “suggests that the decadal changes in stratospheric water vapor have the potential to affect recent climate.”

Observed changes in stratospheric water vapor. Solomon et al./Science.

The authors conclude the paper by saying: “This work highlights the importance of stratospheric water vapor for decadal rates of warming based directly upon observations, illuminating the need for further observations and a closer examination of the representation of stratospheric water vapor changes in climate models aimed at interpreting decadal changes and for future projections.” In other words, we need to improve our theoretical knowledge, gather better data, and make more changes to those inaccurate climate models.

Once again our limited understanding of the mechanisms that control Earth's climate is revealed. Here is a plausible explanation as to why the period from 1980 to 1999 was one of noticeable warming, and why since 2000 things have leveled off or even cooled down a bit. Because the mechanisms that link water vapor to temperature regulation are complex and not well understood the climate change clique concentrated on CO₂—and it has become obvious that treating CO₂ as a form of planetary thermostat is simply not a viable explanation. It is no wonder that the IPCC's carbon dioxide centric climate models didn't get recent temperature swings right. To steal a phrase from American politics, “It's the water vapor, stupid!”

Be safe, enjoy the interglacial and stay skeptical.