Alan, this is somewhat long-winded, please have patience.
Some people seem to struggle with the simple problem of “Global Warming”, which is; over the last 150 years, carbon dioxide (CO2) concentrations in the atmosphere have risen from 280 to nearly 380 parts per million (ppm). This fact is due in most part to human activity and is so well established in scientific literature that one rarely sees it questioned from scientists today. It is quite reasonable for the general public to ask how we know this – however, it is very disingenuous for certain individuals to answer their questions or espouse on topics that one is not expert in – it is too important an issue.
We do understand, to at least 95% confidence levels (no scientist will give you 100%) the real problem. The general public and some politicians may want 100% certainty before they act, but if this is expected – nothing will ever get done.
One simple way that we know that human activity is responsible for the increased CO2 is by looking at historical records. Since the industrial revolution, we have been burning fossil fuels and clearing/burning forested and grassed land at an unprecedented rate, and these processes convert organic carbon into CO2. Careful accounting of the amount of fossil fuel that has been extracted and combusted, and how much land clearing has occurred, shows that we have produced far more CO2 than now remains in the atmosphere. The roughly 500 billion metric tons of carbon we have produced is enough to have raised the atmospheric concentration of CO2 to nearly 500 ppm. The concentrations have not reached that level because the ocean and the terrestrial biosphere have the capacity to absorb some of the CO2 we produce. However, it is the fact that we are producing CO2 faster than the ocean and biosphere can absorb it that explains the observed increase.
The most convincing arguments for scientists regarding anthropogenic global warming (AGW) are based on isotopes and oxygen decreases in the atmosphere. Carbon is composed of different isotopes, 14C, 13C and 12C. 12C is the most common. 13C is about 1% of the total.
CO2 produced from burning fossil fuels and clearing or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases.
Isotope geochemists have developed time series of variations in the 14C and 13C concentrations of atmospheric CO2. One of the methods used is to measure the 13C/12C in tree rings, and use this to infer those same ratios in atmospheric CO2. This works because during photosynthesis, trees take up carbon from the atmosphere and lay this carbon down as plant organic material in the form of rings, providing a snapshot of the atmospheric composition of that time. If the ratio of 13C/12C in atmospheric CO2 goes up or down, so does the 13C/12C of the tree rings. This isn’t to say that the tree rings have the same isotopic composition as the atmosphere – as noted above, plants have a preference for the lighter isotopes, but as long as that preference doesn’t change much, the tree-ring changes will track the atmospheric changes.
Sequences of annual tree rings going back thousands of years have now been analysed for their 13C/12C ratios. Because the age of each ring is precisely known we can make a graph of the atmospheric 13C/12C ratio vs time. What is found is at no time in the last 10,000 years are the 13C/12C ratios in the atmosphere as low as they are today. Furthermore, the 13C/12C ratios begin to decline dramatically just as the CO2 starts to increase – around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning. Furthermore, we can trace the absorption of CO2 into the ocean by measuring the 13C/12C ratio of surface ocean waters. While the data are not as complete as the tree ring data (we have only been making these measurements for a few decades) we observe what is expected: the surface ocean 13C/12C is decreasing. Measurements of 13C/12C on corals and sponges (whose carbonate shells reflect the ocean chemistry just as tree rings record the atmospheric chemistry) show that this decline began about the same time as in the atmosphere; that is, when human CO2 production began to accelerate in earnest.
In addition to the data from tree rings, there are also of measurements of the 13C/12C ratio in the CO2 trapped in ice cores. The tree ring and ice core data both show that the total change in the 13C/12C ratio of the atmosphere since 1850 is about 0.15%. This sounds very small but is actually very large relative to natural variability. The results show that the full glacial-to-interglacial change in 13C/12C of the atmosphere (which took many thousand years) was about 0.03%, or about 5 times less than that observed in the last 150 years.
On time-scales of ~100 years, there are only two reservoirs or carbon sinks that can naturally exchange large quantities of CO2 with the atmosphere: the oceans and the land biosphere (forests and soils). The mass of carbon must be conserved (conservation of mass and energy). If the atmospheric CO2 increase was caused, even in part, by carbon emitted from the oceans or the land, we would measure a carbon decrease in these two reservoirs – it hasn’t.
Both fossil fuel burning and biosphere respiration consume oxygen and reduce 13C as they produce CO2, but the exchange of CO2 with the oceans has only a small impact on atmospheric oxygen and 13C. The measure of atmospheric CO2 increase together with oxygen or 13C decrease gives the distribution between the different reservoirs.
All the estimates show that the carbon content of the oceans is increasing by ~ 2±1 PgC every year (current burning of fossil fuel is ~7 PgC per year). One method is able to go back in time and shows that the carbon content of the oceans has increased by 118±19 PgC in the last 200 years. There is some uncertainty about the exact amount that the oceans have taken up, but not about the direction of the change. The oceans cannot be a source of carbon to the atmosphere, because we observe them to be a sink of carbon from the atmosphere.
What about the land biosphere? We know that deforestation has contributed to the increase in atmospheric CO2. Yet because carbon needs to be conserved, observations of the carbon increase in the atmosphere and the oceans combined with estimates of fossil fuel burning tell us that deforestation has been largely compensated by enhanced growth by the land biosphere. For example, during 1980 to 1999, fossil fuel burning was 117±5 PgC, and the carbon increase in the atmosphere and the oceans were 65±1 and 37±8 PgC, respectively. Thus that leaves 15±9 PgC that has been taken up by the land. This 15±9 PgC includes deforestation (and other land-use changes) which reduced the land biosphere by 24±12 PgC, and an additional land uptake of 39±18 PgC in response to elevated CO2 and climate changes (Sabine et al. 2004). Here also there is some uncertainty about the exact amount, but there is no uncertainty that the land biosphere has taken up a quantity of CO2 that is roughly equivalent to the deforestation.
Why are the ocean and land taking up carbon, when we know that warming of the oceans reduces the solubility of CO2 and warming of the land accelerates bacterial degradation of the soils? The answer is that warming is not the only process that influences the oceans and land biosphere. The dominant process in the oceans is the response to increasing atmospheric CO2 itself. If the oceans had not warmed, they might have taken up even more carbon, although we cannot say for sure because warming may have other impacts, for example on marine biota. On land, bacterial degradation of the soils may have increased in response to warming, but for the moment this effect is smaller than the land response to other processes (for example fertilization by CO2 and nitrogen, changes in precipitation, etc).
Is this consistent with what we know of the glaciations? Yes. During glaciations, the balance of processes was very different. Cooling and other climate changes occurred first. The response of the oceans and land biosphere to climate caused the atmospheric CO2 to decrease, which caused more cooling. During glaciations, there were no external changes in atmospheric CO2 and the oceans and land biosphere responded primarily to climate change. In the last 200 years, there have been large changes in atmospheric CO2 as a result of human activity, and the oceans and land biosphere respond primarily to rising CO2.
The discussion of climate change in “public” is often completely at odds to the discussion in the scientific community (in papers, at conferences, workshops etc.). In public discussions there is often an emphasis on seemingly simple questions that, at first sight, appear to have profound importance to the question of human effects on climate change. In the scientific community however, discussions about these “simple” questions are often not, and have subtleties that rarely get publicly addressed. So I repeat, it is very disingenuous for certain individuals to answer “general concerns” or espouse on topics that one is not expert in.
One such question is the percentage of 20th Century warming that can be attributed to CO2 increases, and the resultant increase in temperatures, that you allude to in your post and infer in it to Mars.
First of all, 'attribution' in the technical sense requires a certain statistical power (i.e. you should be able to rule out alternate explanations with some level of confidence). This is a stricter measure than the word in common phrasing would imply (another example of where popular usage and scientific usage of a term might cause confusion). Secondly, attribution (in the technical sense) of an observed climate change is inherently a modelling exercise. Some model (physical, biological, chemical, etc) of whatever complexity, must be used to link cause and effect – simple statistical correlations between a forcing and a (noisy) response are not sufficient to distinguish between two potential forcings with similar trends. Given that modelling is a rather uncertain business, those uncertainties must be reflected in any eventual attribution. It certainly is an important question whether we can attribute current climate change to anthropogenic forcing – but this is generally done on a probabilistic basis (i.e. anthropogenic climate change has been detected with some high probability and is likely to explain a substantial part of the trends – but with some uncertainty on the exact percentage depending on the methodology used – hence 90-95% certainty of AGW and not 98% (at least so far).
For the case of the global mean temperature however, we have enough modelling experience to have confidence that, to first order analysis, global mean surface temperatures at decadal and longer timescales are a reasonably linear function of the global mean radiative forcings. This result is built in to simple energy balance models, but is confirmed by more complex ocean-atmosphere coupled models and our understanding of long term paleo-climate change. With this model implicitly in mind, we can switch the original simple question regarding the attribution of the 20th century temperature response to the attribution of the 20th century forcing – i.e. what is the percentage attribution of CO2 to the 20th century forcings?
This is a subtly different problem of course. For one, it avoids the ambiguity related to the lags of the temperature response to the forcings (some decades as you quite rightly have picked up on), it also assumes that all forcings are created equal and that they add in a linear manner, and removes the impact of internal variability (since that occurs mainly in the response, not the forcings). These subtleties can be addressed (and are in the formal attribution literature included in the IPCC technical papers), but here is not the time or place to pursue them.
Next, how can we define the attribution when we have multiple different forcings – some with warming effects, some with cooling effects that together might cancel out? Imagine 3 forcings, A, B and C, with forcings of +1, +1 and -1 W/m2 respectively. Given the net forcing of +1, you could simplistically assign 100% of the effect to A or B. That is pretty arbitrary, and so a better procedure would be to stack up all the warming terms on one side (A+B) and assign the attribution based on the contribution to the warming terms i.e. A/(A+B). That gives an attribution to A and B of 50% each, which seems more reasonable. But even this is ambiguous in some circumstances. Imagine that B is actually a net effect of two separate sources (I'll give an example of this later on), and so B can alternately be written as two forcings, B1 and B2, one of which is 1.5 and the other that is -0.5. Now by our same definition as before, A is responsible for only 40% of the warming despite nothing having changed about the understanding of A nor in the totality of the net forcing (which is still +1).
A real world example of this relates to methane and ozone. If you calculate the forcings of these two gases using their current concentrations, you get about 0.5 W/m2 and 0.4 W/m2 respectively. However, methane and ozone amounts are related through atmospheric chemistry and can be thought of alternatively as being the consequences of emissions of methane and other reactive gases (in particular, NOx and CO). NOx has a net negative effect since it reduces CH4, and thus the direct impact of methane emissions can be thought of as greater (around 0.8 W/m2, with 0.2 from CO, and -0.1 from NOx). Nothing has actually changed – it is simply an accounting exercise, but the attribution to methane has increased.
Is there any way to calculate an attribution of the warming factors robustly so that the attributions don't depend on arbitrary redefinitions? No. So we are stuck with an attribution based on the total forcings that can exceed 100%, or an attribution based on warming factors that is not robust to definitional changes. This is the prime reason why this simple-minded calculation is not discussed in the literature very often. In contrast, there is a rich literature of more sophisticated attribution studies that look at patterns of response to various forcings.
More useful is a categorisation based on a separation of anthropogenic and natural (solar, volcanic) forcings. This is less susceptible to rearrangements, and so should be less arbitrary and has been preferred for more formal detection and attribution studies.
What does this all mean in the real world? The total forcing from 1750 to 2000 is about 1.7 W/m2 (it is slightly smaller for 1850 to 2000, but that difference is a minor issue). The biggest warming factors are CO2 (1.5 W/m2), CH4 (0.6 W/m2, including indirect effects), CFCs (0.3), N2O (0.15), O3 (0.3), black carbon (0.8), and solar (0.3), and the important cooling factors are sulphate and nitrate aerosols (~-2.1, including direct and indirect effects), and land use (-0.15). Each of these terms has uncertainty associated with it (a lot for aerosol effects, less for the GHGs). So CO2's role compared to the net forcing is about 85% of the effect, but 37% compared to all warming effects. All well-mixed greenhouse gases are 64% of warming effects, and all anthropogenic forcings (everything except solar, volcanic effects have very small trends) are ~80% of the forcings (and are strongly positive). Even if solar trends were doubled, it would still only be less than half of the effect of CO2, and barely a fifth of the total greenhouse gas forcing. If we take account of the uncertainties, the CO2 attribution compared to all warming effects could vary from 30 to 40% perhaps. The headline number therefore depends very much on what you assume.
But does the specific percentage attribution really imply much for the future? (i.e. does it matter that CO2 forced 40% or 80% of 20th century change?). The focus of the debate on CO2 is not wholly predicated on its attribution to past forcing (since concern about CO2 emissions was raised long before human-caused climate change had been clearly detected, let alone attributed), but on its potential for causing large future growth in forcings. CO2 trends are forecast to dominate trends in other components (due in part to the long timescales needed to draw the excess CO2 down into the deep ocean). Indeed, for the last decade, by far the major growth in forcings has come from CO2, and that is unlikely to change in decades to come. The understanding of the physics of greenhouse gases and the accumulation of evidence for GHG-driven climate change is now overwhelming - and much of that information has not yet made it into formal attribution studies – thus scientists on the whole are surer of the attribution than is reflected in those papers. This is not to say that formal attribution per se is not relevant – it is, especially for dealing with the issue of natural variability, and assessing our ability to correctly explain recent changes as part of an evaluation of future projections. It's just that precisely knowing the percentage is less important than knowing that the observed climate change was highly unlikely to be natural.
Alan, thank you for your reading, I felt your statements/questions needed a substantive response. At the end of the day, I think it best to leave the science to the scientists (they really do only want to tell it like it really is). I can only suggest be careful with the science –it can bore people, it can confuse or panic them or they can stick their head in the sand – none of which is good for anything.