Carl Mears, who runs the RSS global satellite temperature dataset, has a new paper out, studying trends is atmospheric water vapour from 1988-2017. The data reveals a significant increasing trend in Total Precipitable Water (ocean only) of 1.49% per decade, which he states is confirmation of the effects of anthropogenic global warming.

Water vapor in the atmosphere is important for studying our changing climate because it is a contributor to the greenhouse effect and the amount of vapor is expected to increase as the planet warms . . . . .

The record clearly shows that the amount of vapor in the atmosphere has been increasing at a rate of about 1.5% per decade over the last 30 years as the planet warms. This is not surprising, since warmer air tends to “hold” more water vapor,but is a nice confirmation of estimates of temperature rise made by other instruments and methods.

1 Introduction

Understanding the long-term changes in water vapor content on a global scale is critical for assessing human induced climate change. Changes in water vapor, which are expected to track increases in global temperature (Trenberth et al., 2005; Wang et al., 2016) lead to a substantial positive feedback mechanism, and well as directly influencing climate parameters that impact human and natural ecosystems, such as rainfall, evaporation and surface humidity. For these reasons, water vapor has been identified as an essential climate variable by the Global Climate Observing System (GCOS)(GCOS-Secretariat, 2015). Over the world’s oceans, satellite borne microwave radiometers have been monitoring the total precipitable water (TPW) since late 1987 (Wentz, 1997).

But is it, I wonder? Here is the image of how that trend looks like when viewed regionally.

To me, this is uncannily similar to water surface temperature data as measured during a powerful El Nino. I find it very difficult to dissociate the increase in total precipitable water therefore, with the regional increases in Pacific tropical surface water temperatures which are associated with powerful El Ninos. As we have seen, powerful El Ninos have increased considerably in strength and frequency since the beginning of the 1980s, in turn contributing to mass coral bleaching events, which have also been attributed to man-made global warming.

It appears to me that yes, warming is responsible for increasing amounts of water vapour in the atmosphere, but that this warming is intimately associated with the regional ocean surface warming which has manifested as increasingly powerful El Ninos (1982/83, 1997/98, 2015/16 – plus lesser events in between) since the early 1980s. So unless you attribute the occurrence of more frequent and more powerful El Nino events to GHGs, you are left with the conclusion that the increase in precipitable water vapour measured over the last 30 years is largely due to natural causes.