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Scope of This Post: The role of water and carbon di-oxide (CO₂) - the two main actors - in global warming has generated much controversy and confusion.
The subject of global warming (or climate change) is controversial because
(a) the science is complex,
(b) most serious damage won't be felt for a few decades,
(c) outcomes depend on what actions are taken now and in the near future;
(d) The situation is further compounded by deliberate attempts by vested interests actively spreading misinformation.
This post is prepared at two levels;
The first part is suitable as an outreach feature where I explain that while water vapour is responsible for the greatest amount of warming, changes in the atmospheric concentration of CO₂, through feedback loops, determine the amount of water in the atmosphere and thus control the extent of the greenhouse effect - CO₂ is the control knob.
In part 2, we shall compare the role of water and CO₂ in global warming in more detail. The science of how radiation energy interacts with water and CO₂ is well understood; however, predictions of global climate change still suffer from an incomplete understanding of the ways in which earth's feedback loops respond to changes brought about by changing concentrations of the greenhouse gases (GHGs). Our understanding of such feedbacks has greatly improved over the past few decades. Most outcomes point to the predictions erring on the side of caution - many observations point to a more severe climate change than predicted.
Part 1: CO₂ is the Control Knob: The earth receives radiation energy from the Sun over a range of wavelengths. In turn, the earth radiates infra-red radiation from its surface. (See Appendix for discussion of black-body radiation). The earth's atmosphere acts like a blanket in preventing some of the radiation emitted by the earth from escaping to outer space. Our planet is 33⁰C warmer than it would be without the atmosphere - so-called greenhouse effect (without the atmosphere, earth's mean temperature will be -18⁰C). (For background reading please see: Making Sense of our Climate Change: 3. The Science of Global Warming)
Water vapour is the most abundant (~ 1%) heat-trapping gas in the atmosphere and detailed scientific analysis confirms that it keeps the earth warmer by about 20⁰C (responsible for about 60% of the greenhouse effect). CO₂ (~0.04%) and other greenhouse gases (GHGs) contribute about 25%. There is some remaining uncertainty in the role of clouds, but they are deemed to trap about 15% of the earth's radiation from escaping into space.
There are three points to note:
(1) Water vapour is a condensable gas - its amount in the atmosphere is fixed by the earth's temperature - any excess water vapour returns to earth via rain or snow within a short time period of a few days. Other GHGs stay in the atmosphere for much longer (CO₂ stays for several hundred years) and, unlike water, their amounts and hence the warming effects accumulate independent of conditions on earth.
(2) Most water in the atmosphere is found in the first 3 km. 99% of atmospheric water is found in the troposphere - below about 12 km altitude (see slide A7 in the appendix). CO₂ and other GHGs mix much more uniformly throughout the atmosphere. Water plays a minor role in the absorption of outgoing radiation above the troposphere - for altitudes from about 12 to 100 km.
(3) Water vapour does not absorb outgoing radiation in the wavelength range from 8 to 15 microns while CO₂ and O₃ have strong absorption bands in this range. To a good extent, water vapour and other GHGs complement each other in trapping outgoing radiation in different parts of the spectrum.
Before we started burning fossil fuels in large quantities, atmospheric water vapour provided (and continues to provide) a steady contribution to warming by trapping ~60% of the radiation emitted by the earth. The increase in atmospheric greenhouse gases - CO₂, CH₄ & N₂O - over the past 200 years has changed the dynamics of the absorption of outgoing infra-red (IR) radiation. To a large extent, this increase has been responsible for mediating additional global warming that we are witnessing today. CO₂ is more abundant and longer lasting than other GHGs and is the most important influence in the observed additional warming over historic levels during the past several decades.
However, many news articles, without mentioning the significant role of water vapour, claim that carbon di-oxide (CO₂) is the most important GHG and convey the impression that CO₂ alone is responsible for global warming. This is obviously not correct. The unfortunate misrepresentation has engendered increased scepticism about climate change. In turn, this has made it more difficult for the public and politicians to accept the highly damaging long-term predictions based on sound scientific analysis and take effective timely actions. It is important that the science of global warming is shared more accurately with the wider public whose lives will be seriously impacted in not too distant future .
The science of how radiation interacts with atmospheric constituents is well understood. Also, since 1969 satellites and ground based instruments have been providing good quality observational data. It is possible to calculate, with good confidence, how the earth energy budget looks like (slide A4), and how it might be affected by changes in the concentration of particular atmospheric constituents. Some uncertainty remains about the contribution of clouds - cloud cover is highly changeable and depending on their type, clouds can cause warming (by trapping earth's outgoing radiation) or cooling (by reflecting Sun's incoming radiation back to space). On the global level, it is estimated that the two effects roughly cancel each other with the clouds causing a small amount of net cooling.
The Three Actors: Earth's temperature is dertermined by three actors - The Sun, the Earth and the atmosphere.
The Sun is the primary source of all energy and the top of Earth's atmosphere (TOA) receives energy from the Sun at a rate of 1362 Wm⁻² (Watt per meter squared) in the wavelength range from 0.15 to 4 µm with a maximum around 0.5 µm (1 µm = 10⁻⁶ m = 1000 nm). Over the past 50 years, total solar irradiance, measured at TOA, has been constant to within 1 Wm⁻².
On a clear day, 40% of the solar radiation received at the earth's surface is visible radiation (0.4 to 0.7 µm) while 51% is infra-red (IR) in the spectral range 0.7 to 4 µm. Most of the ultraviolet (< 0.4 µm) radiation is absorbed by ozone in the stratosphere. We shall call the Sun's radiation (wavelength < 4 µm) the short-wave radiation (SWR).
The Earth warms by receiving SWR but radiates energy in the IR between 2 and 70 µm wavelength range with a maximum around 10 µm. On its way to outer space, energy radiated from the earth passes through the atmosphere and interacts with its constituents. We shall call the radiation emitted by the earth (wavelength > 4 µm) the long-wave radiation (LWR).
The Atmosphere is a collection of stratified gases and extends to an altitude of roughly 100 km above the earth's surface (see slide A7). It plays a complex role in interacting with SWR and LWR - gaseous layers reflect, absorb, re-radiate the radiation passing through the atmosphere. Interactions in the atmosphere comletely change the nature and amount of longwave radiation going to outer space - this is discussed next.
Earth's Energy (Radiation) Budget: The following slides show what happens to the incoming radiation from the Sun and outgoing radiation from the Earth and the role of the atmosphere. Only a small fraction of the earth's radiation (mainly in the atmospheric window from 8 to 14 micron) goes directly to space - almost 90% is first absorbed by the atmosphere which then emits it in all directions (see slide 2 and A4)
Slide 1:
Slide 2:Slides 1 and 2 explain how earth's energy (radiation) budget (EEB or ERB) is determined. In slide 2, the width of arrows indicate (for guidance only) the amount of energy transferred (for quantitative description see slide A4 in the appendix). If ISWR is the net incoming shortwave radiation absorbed by the earth and OLWR is the net outgoing longwave radiation escaping to space from the top of the atmosphere, then
If ISWR = OLWR; Earth and Atmosphere temperature remains unchanged
If ISWR > OLWR; Earth gains energy; temperature increases - earth warms
If ISWR < OLWR; Earth loses energy; temperature decreases - earth cools
Notice that the earth and the lower atmosphere are in thermal equilibrium and any shift in earth's temperature also changes the temperature in the troposphere in the same sense.
Slide 3: The major contributors to Earth's radiation budget. This is an iconic image that is full of interesting information. Essentially what we are seeing here is the way individual gases absorb incoming and outgoing radiation.
OLWR (shown blue in slide 3) is what we call the atmospheric window and is the radiation leaking into outer space. Water and other GHGs absorb longwave radiation and re-emit in all directions; see slide 2). Slide 4 shows the relative roles of water and CO₂ in greater detail - notice the broad absorption band of CO₂ at 15 microns absorbs radiation where water absorption is not saturated. But water vapour is seen as the most important absorber of LW radiation:
Slide 4:
Clouds contain liquid water/ice crystals and can absorb radiation in the atmospheric window, retaining even more energy in the atmosphere, some of which is radiated back to earth. That is why local temperatures are higher on cloudy days.
Water vapour and clouds are responsible for 75-80% of the greenhouse effect - CO₂ and other GHGs for the remaining 25-20%. The maximum amount of water vapour that can be present in the atmosphere depends on its temperature - the earth temperature determines the amount of water vapour in the atmosphere.
Starting with current levels of CO₂, if we were to remove all of it then overall greenhouse effect will be weaker by about 20% and the equilibrium temperature of the earth & atmosphere will set lower. This will reduce the amount of water vapour that can be sustained in the atmosphere (see Appendix - water vapour pressure). With less water vapour, decreased amount of outgoing longwave radiation will be trapped. Further cooling will result. The atmosphere will hold even less water vapour. Also more surface area of the earth will be covered by highly reflecting ice and less solar energy (ISWR) will be absorbed by the earth. The feedback loop will continue until we reach a state when the earth is frozen down to the tropics. Slide 5 shows the projections of what will happen if all CO₂ is removed from the atmosphere:
Slide 5:
The presence of GHGs is vital for the earth to remain at habitable temperatures. However, if GHGs amounts increase then they will trap more of the outgoing radiation and result in additional warming of the atmosphere and earth surface. Warmer atmosphere will be able to hold more water vapour. Extra water will further strengthen the greenhouse effect. This is the loop we are currently in when CO₂ concentration has been increasing by 2+% per annum for several decades.On the current trajectory of rising CO₂, by the year 2100, the global mean temperatures could see a rise of 3+⁰C over pre-industrial levels. This will be most unwelcome and will bring widespread misery for a good part of the global population. It is important to control atmospheric GHG levels urgently.
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Part 2: More Detailed Analysis of Global Warming:
In part 1, it was established that without CO₂ and other noncondensing greenhouse gases, the terrestrial
greenhouse effect would quickly collapse and plunge the global climate into an icebound Earth within a short time interval of a few decades. In Part 2 here, we shall analyse the situation in more detail.
Earth's net energy budget is profoundly influenced by what happens in the atmosphere - interestingly, the major components of the atmosphere (78% N₂ and 21% O₂) play practically no part - they are almost perfectly transparent to visible and infra-red radiation in the wavelength range of interest (0.4 to 70 µm). Water vapour, ozone and CO₂ are responsible for most of the absorption and re-emission of radiation. Slides 6 and 7 show how these three gases affect incoming SWR and outgoing LWR:
Slide 6:
Slide 7:
Radiative Forcing: Atmospheric constituents change the balance between incoming solar radiation and outgoing longwave radiation and destabilize earth's thermal equilibrium. Radiative forcing (RF) of an atmospheric constituent is a measure of how the energy balance of the earth-atmosphere system is influenced by it. RF is measured in Wm⁻².
For example, the longwave RF (LWRF) due to the presence of an atmospheric constituent is the difference between earth's surface radiation and outgoing longwave radiation (OLWR) at the top of the atmosphere. LWRF for various gases may be calculated from the data in slide 7.
Slide 8:
Slide 9 shows how radiation forcing due to some of the most important constituents of the atmosphere (climate drivers) has changed between 1750 and 2011 - CO₂ is the most important contributor to RF. Not included in slide 9 are water vapour and clouds. Even though they contribute most to the greenhouse effect (slide 4), as mentioned before they are manifestation of feedbacks in the climate system and are not primary drivers of climate change (see section on feedbacks below).
Slide 9:
Slide 9 lists a number of ways human activity has contributed to climate forcing since the first industrial revolution. Main contributors are - greenhouse gas emissions, particle pollution (aerosols) and deforestation (changes how earth's surface absorbs and reflects sunlight). Changes in nature also cause climate forcing and have been responsible for many historic climate extremes (ice ages and interglacial periods). Large volcanic eruptions inject light-reflecting particles into the stratosphere and also emit copious amounts of carbon di-oxide gas. Aerosols in the stratosphere have long residency times and cool the earth by reflecting sunlight back to space. Milankovitch cycles refer to small slow variations (happening over thousands of years) in the shape of the earth orbit around the Sun and changes in the tilt of the axis of earth rotation in space. These can affect the amount of sunlight reaching the earth and start a cooling or warming trend. Over the past 200 years, natural climate drivers have only had a minor effect on global climate (slide 9)
Recent data confirms (slide 10) that the amount of GHGs in the atmosphere is continuing to increase with corresponding increase in RF and global mean temperature. (f
or more details click here, here) Warmer atmosphere holds more water vapour and this makes further strong contribution to greenhouse effect.
Slide 10:
An interesting observation is about the amount of aerosol particles present. Nations are taking outdoor air pollution very seriously and working to reduce aerosols in the environment. Slide 9 shows that aerosols have negative radiation forcing - they have a net cooling effect - because aerosols scatter some sunlight back to space and also provide nucleation centres for cloud formation. Paradoxically, steps taken to improve the global health by reducing aerosols in the environment may result in making greenhouse effect stronger.
On our planet, all systems are interconnected and continuously exchange information, material and energy. Any change in one system has a cascading effect on other systems. In the climate web, temperature, precipitation and wind are the primary drivers of achieving some sort of equilibrium within the systems. Slide 11 gives a partial list of such interconnections and indicates how feedback loops may become active in response to climate change and may affect the way climate itself is changing.
The most important and extensive is the hydrological system that operates on the three states of water (solid ice, liquid water and water vapour). See slide 11 for more details on feedback loops.
Slide 11:
Timescales on which different feedbacks operate differ widely - fast feedbacks are of great relevance to us as they can cause large changes over human lifespan. Slow feedbacks affect the climate system over a much longer time periods of centuries to many thousands of years. We shall look at some fast feedbacks in the following:Albedo Change: Bright white ice and snow reflect a large portion of the Sun's energy that hits the earth's surface - they have a high albedo (>70%). When sea ice and snow melt, they leave behind a darker surface of ocean water or soil that has a much lower albedo (~6%). The result is that the surface retains more of the incident solar energy and warms up. The mean global temperature increases to melt even more ice and snow that results in trapping even more energy. Water Vapour Change: The amount of water vapour that the atmosphere can hold depends on the atmospheric temperature. For each degree increase in temperature, the water vapour pressure increases by about 7% and represents the extra amount of water in the atmosphere due to temperature rise. The extra water vapour increases absorption of outgoing longwave radiation resulting in more warming. This in turn increases the amount of water vapour in the atmosphere further.
Permafrost Melting: Permafrost are regions where earth has been frozen year round for thousands of years. Permafrost hold very large amounts of methane. Global warming is causing permafrost to melt and release trapped methane. Methane is a powerful GHG and causes an intensification of the greenhouse effect. The resulting warming will melt more permafrost and still more methane will be released to the atmosphere setting up a positive feedback loop.
Slide 12:
Slide 12 summarises the situation and also lists some of the negative effects of climate change. Many of the effects are already being felt, and it is expected that certain slow feedback loops, already active, will keep global warming intact for centuries.
Slow Feedback Loops: Melting of ice sheets is an example of slow feedback loops. Greenland and Antarctica are covered by ice sheets that are up to 2 km thick. Greenland ice sheet (GIS) had melted in the past during interglacial periods and holds enough water to raise the sea level by 7 metres. The time scale for GIS to melt is estimated at several thousand years - however, if the global temperatures rise more rapidly then we might start to see significant sea level rise due to GIS melting much sooner. There is good evidence that GIS had melted about 400,000 years ago when carbon di-oxide levels were not as high as they are now.
Hydrological Feedback Loop: Water based feedback is unique in that it enhances greenhouse effect by a much greater amount than the original radiative forcing by the GHGs. Water vapour is the most important actor in causing global warming. However, the presence of water vapour in the atmosphere is only possible because GHGs (mainly CO₂) are already there and on their own GHGs warm the earth enough for water to vaporise in sufficient quantity to eventually become the main greenhouse agent. (How vaporisation of water depends on ambient temperature is discussed in the Appendix, see slide A6).
How the amount of atmospheric water vapour affetcs its ability to cause warming has been studied in a clever way by using climate models by Andrew Lecis's group at NASA. Without changing the concentration of GHGs (i.e. keeping the amount of CO₂ constant), in various scenarios, first they doubled the amount of water in the atmosphere and then set it to near zero. The results clearly show that the only viable concentration of water in the atmsphere is what is allowed by the prevailing temperature. I have redrawn some of their results in a simplified form to demonstrate that while water is a potent absorber of infrared radiation, it is the GHGs that control the amount of atmospheric water. Too little or too much water than what thermodynamics allows is returned to normal levels within a matter of days. Simplified plots are shown in slides 13 and 14 - for full details, please see the original paper.
Slide 13:
In the slide, the continuous red line shows the scenario when water level was doubled in the atmosphere near the earth surface. The atmposphere is unable to hold the extra moisture and through precipitation (rain and snow) water levels return to normal (as allowed by thermodynamics - see appendix). The dashed red line shows what happens when the water levels are doubled at the tropopause (about 12 km above the earth's surface). Mixing of gases takes somewhat longer due to limitations of atmospheric dynamics causing a time delay of a few days but the extra atmospheric water again precipitates as in the first case.
Blue lines show the results when water level were zeroed at the surface or at the tropopause. The deficit of water is made up by evaporation from the oceans to return water levels to the equilibrium levels - again we notice the delay in the second case due to the limitations of vertical mixing efficiency (transporting evaporated water to the upper layers of the atmosphere).
Slide 14:
Slide 14 shows what happens to the longwave flux at the top of the atmosphere in the two scenarios. By doubling water vapour, we introduce much greater absorption of LW radiation in the atmosphere and less radiation flux reaches TOA - effectively greenhouse effect is enhanced by about 17 Wm⁻² for a short period but is rapidly corrected as water levels reset to normal (slide 13). When all water is removed, the most important absorber is no longer present and much more of the LW flux reaches TOA - the greenhouse effect is weakened by almost 50 Wm⁻², but again resetting to normal values very quickly.The conclusion from this study is that amount of atmospheric water is determined by the mean global temperature that is initially set by the atmospheric concentration of GHGs (mainly carbon di-oxide).
APPENDIX
Blackbody Radiation: All bodies radiate energy in the form of electromagnetic waves - the energy radiated per second (power) dependes on the surface area and temperature of the body (Stefan-Boltzmann Law) and the range of wavelengths emitted is given by the Planck distribution curve. Note that the temperature is the absolute temperature of the body in unit of Kelvin and is related to degree centigrade as: T (Kelvin) = T (⁰C) + 273. The situation is demonstrated in Slide A1 below:
The total energy radiated per second from a given area changes rapidly with the temperature (T) of the body - as fourth power of T. Doubling the temperture increases the energy emitted by a factor of 16. Sun's surface is about 20 times hotter than the earth; and the Sun radiates 20⁴ or 160,000 times more energy per unit area. Also the Sun is much bigger than the earth.
The wavelength at which peak energy is radiated depends on the temperature of the emitting body. There is a simple relation called the Wien's displacement law that tells us where the maximum intensity of the Planck curve is:
Wavelength (max intensity) in microns = 2898/Temperature in Kelvin
Remember: 1 micron = 10⁻⁶m = 0.001 mm; and T (Kelvin) = T(⁰C)+273
We can summarise the discussion by plotting the spectral distribution of radiation from the Sun and the Earth. In Slide A2, the Sun's radiation intensity has been scaled down to compare its shape with the earth's emitted radiation. The main take away is that the Sun radiates in regions around the visible wavelengths while the earth's radiation is totally in IR extending from 4 to 70 micron wavelengths.
Slide A2:
From the well understood properties of gases and how they interact with radiation, it is possible to calculate earth's energy budget (EEB) with good confidence. To keep the task manageable, the atmosphere is treated as consisting of a small number of layers:
Slide A3: Earth Energy Budget (results from models):
The properties of the atmospheric layers are chosen to reproduce the measured energy fluxes at top of the atmosphere (TOA) and current climate parameters. Slide A4 shows the result for EEB assuming a 3 layer atmosphere. These comprehensive calculations (KT97) were performed in 1997 and have since been revised with minor changes in the results. I show the original results - for recent updates (TFK09, Trenberth Diagrams, Lacis). KT97 and Lacis describe the details of how properties of the layers are chosen and all other relevant information.
Slide A4:
In the diagram above, the incoming energy is equal to the outgoing energy both at the top of the atmosphere (342 Wm⁻²) and at the earth's surface (492 Wm⁻²) - this represents a state of equilibrium and the temperature of the earth and its atmosphere will stay constant. Notice that the total energy flux of 492 Wm⁻² arriving at the earth's surface is much greater than 342 Wm⁻² arriving at TOA because atmospheric gases absorb and re-emit substantial amount of LWR emitted by the earth back towards it. Slide A5 (this is the same as slide 3 in the main text - I show it for completeness) shows the major contributors to Earth's radiation budget.
Slide A5:
Vapour Pressure of Water: The equilibrium between water vapour and liquid water in any system depends on the temperature of the system. The equilibrium (also called saturated) water vapour pressure (VP) increases as temperature goes up. The Clausius-Clapeyron equation describes how the VP changes with temperature.
Slide A6: 
The graph in slide A6 shows the saturation vapour pressure (VP) of water as a function of temperature. If the amount of water in gaseous state is greater than VP then the excess water will condense to bring VP to the equilibrium value. This is the reason that most water is found in the warmer part of the atmosphere with the troposphere containing 99% of the atmospheric water (slide A7). Also, if the amount of water in gaseous state is lower than the saturation VP for the temperature, then some water will evaporate from the liquid state to bring the system to thermal equilibrium as demanded by the Clausius-Clapeyron equation.Slide A7:
It is straightforward to understand the variations shown in slide 7. The earth warms by receiving solar radiation; the increase in surface temperature causes water to evaporate - water vapour has much greater internal energy than liquid water (latent heat of vaporization) and evaporation represents net energy transfer from earth to the atmosphere (slide A4). Gases at the earth surface also heat up and rise (convection) and carry energy to the atmosphere. As water and gases rise, they expand and cool until their temperature is equal to that of gases at higher altitudes (of the order of -65⁰C) - this layer is called the tropopause that is about 10 to 12 km above the earth's surface. Beyond tropopause temperature rises again and cooler gases of the troposphere cannot rise higher and can only move sideways. There is very little mixing of gases between the troposphere and stratosphere. Stratospheric ozone layer is at about 20 km altitude - ozone absorbs ultraviolet radiation energy from the Sun and warms the rather tenuous atmosphere quite efficiently, so much so that the temperature warms up to just below 0⁰C at the top of the stratosphere. Because the density of gases is very low in the stratosphere, it takes only a small quantity of increase its temperature.
It might be useful to mention some interesting facts about water in the atmosphere. Of all the water on earth, 0.001% is in the atmosphere - enough to cover the earth's surface by 2.5 cm (= 1 inch)! Each gram of water that evaporates reduces the energy on earth by 2500 J (600 calories). This energy is released to the atmosphere when water vapour liquefies and falls back on earth as rain, heating the lower troposphere.
Some relevant information about this post is that water vapour is such a good absorber of outgoing longwave radiation that it absorbs most of the radiation emitted by the earth (except in the atmospheric window) very quickly and reradiates it at the prevailing temperature of the troposphere at that altitude - in fact the measured effective temperature of the outgoing radiation at top of the atmosphere is -18⁰C and not 15⁰C (surface temperature of the earth). Additionally, most of the water is in the troposphere where it is the most potent greenhouse gas. Above the tropopause, CO2 and other GHGs provide most of the greenhouse effect with water only playing a minor part in the stratosphere - there is much less absorption of outgoing LW energy beyond the tropopause (discussed in more detail in KT97). See also the very interesting figure here.
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