Water exists in three states on the Earth - in gas (water vapor), liquid (water) and solid (ice/snow) form. Water vapor in the Earth’s lower atmosphere is a powerful greenhouse gas since it absorbs the infrared radiation (or heat energy) that is emitted from the earth’s surface and radiates it back to the surface, preventing it from escaping out to space. Water vapor is just one of a number of greenhouse gases in the Earth’s atmosphere, which also include carbon dioxide, methane, nitrogen dioxide and hydrofluorocarbons. Although abundant in the atmosphere, oxygen and nitrogen are not greenhouse gases. Due to their molecular structure they are unable to absorb and re-emit infrared radiation.
A certain amount of greenhouse effect is necessary to keep the Earth at a habitable temperature. Without it, the Earth would be on average 60 degrees Fahrenheit cooler. However, adding greenhouse gases to the atmosphere increases the surface temperature with potentially serious consequences. As an example, our neighboring planet Venus has a thick atmosphere composed largely of carbon dioxide, an effective greenhouse gas. Its surface temperature is several hundred degrees Celsius hotter than on Earth and it is hotter than Mercury, even though Mercury is closer to the Sun.
Water vapor is the most abundant greenhouse gas in the atmosphere. Importantly, a warmer atmosphere can hold more water vapor. As the atmosphere warms due to climate forcings (whether manmade or natural) the amount of water vapor contained in the atmosphere increases. This increase of water vapor in the atmosphere is a feedback, since it follows the temperature increase and amplifies the initial effect. As atmospheric water vapor increases, the greenhouse effect is amplified, causing greater warming and further evaporation. Note that the reverse is also true. As the planet cools, the ability of the air to hold moisture decreases and therefore the atmosphere cools further.
Water vapor, although abundant, is one of the shortest lived of the atmospheric greenhouse gases. Its lifetime in the atmosphere can be measured in days rather than years or decades. There is a basic physical limitation on the amount of water vapor that can remain in the atmosphere for any given temperature, measured by the saturation vapor pressure. Once the saturation water vapor pressure is reached, water vapor condenses and falls to Earth as rain or snow. Note that the increase in atmospheric water vapor leads to greater and more intense precipitation in given regions (including more snow at the polar regions or in temperate regions). Note that other greenhouse gases, in particular carbon dioxide, stay in the atmosphere for decades and contribute greatly to the long-term warming of the atmosphere.
So how do changes such as higher temperatures and greater amounts of water vapor in the lower atmosphere affect the higher regions of the atmosphere? In brief, the Earth’s atmosphere is composed of five main layers, with differing chemical compositions. Looking at the two layers closest to the Earth’s surface we have the troposphere, which contains most of the Earth’s weather. The average height of the troposphere above the surface is approximately 11 miles in the middle latitudes. It is higher in the tropics up to 12 miles and shallower near the poles at around 4.5 miles. Temperatures in this layer decrease with altitude. The stratosphere sits above the troposphere from about eight to thirty miles above the Earth’s surface, and contains the protective ozone layer. Temperatures in this layer increase with altitude. Between these two layers we have the tropopause, and is the demarcation region between when temperatures start to rise again with altitude.
We have seen that both water vapor and temperatures have increased in the troposphere over the past four decades of satellite measurements. Recent studies have also started to examine temperature changes and water vapor changes in the stratosphere and its effect on the Earth’s warming. Water vapor enters the stratosphere through the rising of air in the warm and humid tropics, where extremely tall thunderstorms are capable of transporting water vapor into that region of the upper atmosphere. Water vapor can also reach the stratosphere through the chemical breakdown of methane into water vapor and carbon dioxide.
The potency of water vapor as a greenhouse gas is much higher when it is located in the lower stratosphere where temperatures are extremely cold. Greenhouse gases located in cold regions of the atmosphere are more effective at heating the planet because they absorb heat radiation from the Earth's relatively warm surface, but then re-emit energy at a much colder temperature, resulting in less heat energy lost to space and more heat retained by the lower atmosphere and surface.
Accurate global satellite measurements of stratospheric water vapor have only been available since 1991 (although global satellite data has been available since the 1970s). Studies show that stratospheric water vapor increased during the 1990s, followed by a decrease between 2000 and 2009. A study found that the increase in stratospheric water vapor in the 1990s led to a 30% increase in the amount of global warming observed during that decade, and the decrease since 2000 led to a 25% reduction between 2000 and 2009. What is noticeable in these observations is that the changes in stratospheric water vapor occurred mainly in the areas affected by the El Nino Southern Oscillation and near the tropopause, which would suggest the changes are driven by internal variability and are not a dampening feedback.
In summary, while increases of carbon dioxide and other greenhouse gas emissions have led to tropospheric warming near the surface and higher in the troposphere, studies have found that in the last decade or so, there has been a measurable global cooling in the stratosphere. Climate models predict that if greenhouse gases are to blame for heating at the surface, compensating cooling must occur in the upper atmosphere. We need only look as far as our sister planet, Venus, to see the truth of this theory. However, neither the changes in stratospheric water vapor and temperature, nor tropospheric changes in water vapor and temperature are the drivers of change to the Earth’s climate.
Those observed changes follow in response to other factors. A more recent study has attempted to understand the effect of human influence on changes in the composition of the troposphere and stratosphere. While natural factors such as volcanic eruptions and internal variability such as ENSO play their part, a human fingerprint has been detected.
 A climate forcing is an external force that acts as the initial driver to a change in the earth’s climate. Examples include changes in solar radiation, changes in albedo or surface reflectivity, increased greenhouse gas emissions into the atmosphere, or introduction of aerosols such as sulfates or volcanic emissions.
 The El Niño-Southern Oscillation (ENSO) is a naturally occurring phenomenon that involves fluctuating ocean temperatures in the equatorial Pacific. ENSO is known as a dominant force causing variations in regional climate patterns.