The climate science translation guide
by Shaun Chamberlin on September 3rd, 2008
We are all familiar with the concept of climate change, and the need for reduced carbon emissions, but really getting a handle on the scale of the problem can be difficult, thanks to all the confusing terminology.
I looked all over the web for a straightforward comprehensive explanation of terms like CO2equivalent, Global Warming Potential etc. but I couldn’t find it, so eventually I decided to spend some of my time (and the time of many helpful friends and colleagues) on creating one.
I didn’t count on quite how intricate the underlying science is (it became ever clearer to me why there is so much confusion in this area), so the process took some considerable time, but I believe that this post is now something that many will find useful. It has been checked for accuracy by qualified experts.
In order to fully understand the relationship between greenhouse gas emissions and global temperature increase then, we first need to consider the concept of radiative forcing.
The Earth is continually receiving energy from the Sun, and continually losing energy into space (as space is much cooler than the Earth). Radiative forcing is simply the difference (measured in watts per square metre) between the amount of energy received and the amount of energy re-radiated back into space. In other words it is the rate at which the planet’s surface is either warming or cooling. [1]
If the planet were losing energy at the same rate it was gaining it then the radiative forcing would be zero and the temperature would remain stable at its current level – this state is called thermal equilibrium. Since a hotter planet loses more energy into space, the natural system tends to move towards thermal equilibrium.
However, rising greenhouse gas concentrations (measured in parts per million – ppm [2]) in the atmosphere act like an insulating blanket, reducing the rate at which energy can escape into space, and so affecting radiative forcing, which in turn affects the temperature. The rough illustrative graphs below give an idea of these relationships and show the time delay between changes in emissions rates (up or down) and temperature changes. [3]
The graph below shows that if we can bring anthropogenic (human-caused) emissions back down we can stabilise greenhouse gas concentrations and bring radiative forcing back towards equilibrium, but at a higher temperature. [4]
So emissions contribute to greenhouse gas concentrations which in turn contribute to radiative forcing, but it is radiative forcing that determines the rate of change in temperature. Armed with this understanding, the terms below become clearer:
Global warming potential (GWP) is an estimate of how much a given greenhouse gas contributes to Earth’s radiative forcing. Carbon dioxide (CO2) has a GWP of 1, by definition, so a gas with a GWP of 50 would increase radiative forcing by 50 times as much as the same amount (mass) of CO2. A GWP value is defined over a specific time interval, so the length of this time interval must be stated to make the value meaningful (most researchers and regulators use 100 years).
For example, methane has a GWP of 72 over 20 years, but a lower GWP of 25 over 100 years. This is because it is very potent in the short-term but then breaks down to CO2 and water in the atmosphere, meaning that the longer the period you consider it over, the more similar its effect is to that of CO2 alone. [5]
Equivalent carbon dioxide (CO2e) is an estimate of the concentration of CO2 (in ppm) that would cause a given level of radiative forcing. [6]
For example, the IPCC’s[7] latest report in 2007 considered the effects of the main greenhouse gas concentrations currently present in our atmosphere and calculated a CO2e for these of around 460ppm (and rising). This means that (over a defined period) the radiative forcing effect of these gases at current levels is roughly equal to the effect a 460ppm concentration of CO2 alone would cause. This particular CO2e calculation takes into account the six major greenhouse gases considered under the Kyoto Protocol, and so may be labelled CO2e(Kyoto). [8]
However, the orange line in the graphs above represents the total radiative forcing of the planet. This is the important figure – the one that determines the rate of change in Earth’s temperature – and as well as the Kyoto gases it is also affected by other factors such as the effects of sulphate aerosols, ozone and cloud formations. The chart below quantifies the effect of each of these factors, and we can see that a number of them (those coloured blue) are actually negative forcings, which act to reduce the total radiative forcing. Because of these, the equivalent CO2 for all forcings combined - CO2e(Total) - is, thankfully, lower than CO2e(Kyoto). The IPCC’s latest figures give CO2e(Total) as roughly 375ppm. [9]
When we hear scientific debates between stabilisation scenarios of, say, 350ppm, 450ppm or 550ppm it is CO2e(Total) which is under discussion. So this 375ppm is the key number, but it has a far wider margin of error than the others. This is because it is relatively easy to measure the atmospheric concentrations of greenhouse gases, and the GWP of those gases, but considerably more difficult to account for all the effects that contribute to the ultimate CO2e(Total) radiative forcing over a given period. The column in the below chart labelled LOSU stands for the “Level Of Scientific Understanding” of the various forcings, and as we can see it is not universally high. [10]
Radiative forcing is the fundamental issue, but it is easy to see why most discussions revolve only around emissions – not only are CO2 emissions much the largest way in which humanity is changing the planet’s radiative forcing, but they are also easier to understand conceptually and easier to quantify than radiative forcing.
According to the IPCC atmospheric CO2 concentrations were 379ppm in 2005, which coincidentally happens to be close to our best estimate of 375ppm CO2e(Total). Unfortunately this coincidence also creates a good deal of confusion, as it is not always clear which measure an author is referring to – scientists often assume that this is obvious to their audience, and many others do not themselves fully understand the distinctions between CO2, CO2e(Kyoto) and CO2e(Total). [11]
The other source of confusion is that all of the numbers we have discussed are based on evolving science, and many can only be given approximately. For example, these are the IPCC’s given figures for the GWP of methane over 100 years, taken from their last three reports:
1995 - 2nd Assessment Report (SAR): Methane 100 year GWP = 21
2001 - 3rd Assessment Report (TAR): Methane 100 year GWP = 23
2007 - 4th Assessment Report (AR4): Methane 100 year GWP = 25
These changes are entirely appropriate – the values should become more accurate over time as new measurement methods or changes in scientific understanding develop – but it makes it important to check where any figures are sourced from. [12]
Where we are today
So let’s take stock. Below are the latest IPCC figures, which define the situation as it was in 2005:
CO2 = 379ppm (error range: minimal)
CO2e(Kyoto) = 460ppm (error range: 433-477ppm)
CO2e(Total) = 375ppm (error range: 311-435 ppm) [13]
Emissions are still increasing year-on-year (faster than projected in any of the IPCC’s scenarios) and atmospheric CO2 concentrations are currently rising by between 1.5 and 3 ppm each year. They are at roughly 385ppm in mid-2008. It is worth noting that the pre-industrial concentration of CO2 in our atmosphere was 278ppm and did not vary by more than 7ppm between the years 1000 and 1800 C.E. [14]
Global average (mean) temperature has already risen by around 0.8°C since pre-industrial times, and another 0.6°C is still due from past emissions as a consequence of the time-lags in the system explained above. [15]
Ok, that’s it! If you followed everything here you should be well-equipped to consider the scientific discussion of climate change. Indeed, you may find you understand it better than some of those who write and speak about it!
Hopefully this post will provide a resource to aid wider understanding of the changes we are causing to our global climate system and the climate emergency we are facing. Should any inaccuracies come to light I will of course amend them.
This work is part of the Transition Timeline project I am currently undertaking for the Transition Network, which will be published in late 2008. It uses the understanding outlined here to examine the wider context of climate change and peak oil, discusses the options facing our communities and considers the cultural stories which underlie our choices.
Footnotes
1. There is also a warming effect from the geothermal energy at the Earth’s core, but this is sufficiently small and stable that for our purposes we can ignore it.
2. Parts per million is the ratio of the number of greenhouse gas molecules to the total number of molecules of dry air. For example, 300ppm means 300 molecules of a greenhouse gas per million molecules of dry air. Strictly speaking concentrations are measured in parts per million by volume (ppmv), but this is widely abbreviated to ppm. Don’t be confused if some papers refer to ppmv.
3. Emissions are not the sole determinant of atmospheric greenhouse gas concentrations due to the Earth’s natural ‘carbon sinks’ which soak up some of our emissions. Concentrations are not the sole determinants of radiative forcing due to other forcings which will be discussed shortly. The time delay between radiative forcing and temperature increase is caused by the thermal inertia of the planet – it has great mass (with much of the heat initially being used to warm the deep oceans) and therefore takes some time to warm or cool. Of the (equilibrium) temperature increase ultimately produced by a given increase in radiative forcing, only about half manifests within 25 years, the next quarter takes 150 years to manifest, and the last quarter many centuries.
4. These illustrative graphs do not include the effects of climate feedbacks such as carbon sink degradation. Also see: http://tinyurl.com/6y3uqc
5. Figures from IPCC 2007: http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch02.pdf , Table 2.14, p. 212.
More detail on GWP available at: http://en.wikipedia.org/wiki/Global_warming_potential - note that the GWP for a mixture of gases cannot be determined from the GWP of the constituent gases by any form of simple linear addition.
6. There is also a separate but related concept called Carbon Dioxide equivalent. This gives the amount of CO2 that would have the same GWP as a given amount of a given gas (or mixture of gases). It is simply calculated by multiplying the GWP of the gas by the given amount (mass) of gas. For example, over a 100 year period methane has a GWP of 25, so 1 gram of methane has a Carbon Dioxide equivalent value of 25 grams.
In practice, since Carbon Dioxide equivalent is expressed as a mass (grams, tonnes etc.), and Equivalent Carbon Dioxide (CO2e) is expressed as a concentration (usually in parts per million), they are not easily confused, despite the similar names.
7. The IPCC is the Intergovernmental Panel on Climate Change - the body established jointly by the United Nations and the World Meteorological Organisation in 1988 to assess the available scientific evidence.
8. The IPCC considered the so-called ‘Kyoto basket’ of greenhouse gases (GHGs). Under the Kyoto Protocol, signatories committed to control emissions of a ‘basket’ of six GHGs - carbon dioxide, methane, nitrous oxide, HFCs, PFCs and SF6.
The IPCC calculation of CO2e(Kyoto) is detailed by Gavin Schmidt of NASA at: http://www.realclimate.org/index.php/archives/2007/10/co2-equivalents/
9. These negative forcings include the so-called ‘global dimming’ effect. IPCC CO2e(Total) figure: http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf, Notes to table 5.1, p.67
10. Table source: http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf - Figure SPM2, p.4
11. IPCC 2005 CO2 levels: http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf
12. IPCC 2001 figures: http://www.grida.no/climate/ipcc_tar/wg1/248.htm, Table 6.7
1995/2007: http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch02.pdf , Table 2.14, p. 212
13. Error ranges: http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter1.pdf , p.102
14. Up-to-date measurements of atmospheric CO2 concentrations are always subject to revisions, pending recalibrations of reference gases and other quality control checks. Trends and 2008 figure taken from: http://www.esrl.noaa.gov/gmd/ccgg/trends/ (site accessed August 2008)
Pre-industrial CO2 levels from: http://www.noaanews.noaa.gov/stories2005/s2412.htm
15. See footnote [3] above and Climate Code Red, Carbon Equity.
Images
1. Climate-o-meter used (in edited form) with permission from http://www.ageofstupid.net/
2. Radiative forcing illustration used with permission from David Wasdell
3. Indicative climate graph created by author in partnership with David Wasdell, and with assistance gratefully acknowledged from Ben Brangwyn.
4. Indicative climate graph created by author in partnership with David Wasdell, and with assistance gratefully acknowledged from Ben Brangwyn.
5. Radiative forcings table from: http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf - Figure SPM2, p.4
6. Indicative climate graph created by author in partnership with David Wasdell, and with assistance gratefully acknowledged from Ben Brangwyn.
September 3rd, 2008 at 11:55 pm
Excellent, thankyou. And gratifying that I’m some way up the spectrum of climate change understanding. (and a little further now than this morning). Cheers
September 5th, 2008 at 8:55 am
An impressive bit of sorting out, but I am guessing that it is still far too complex for most people to sort out the message coming from science … which is potentially disastrous since it perhaps will require the people to insist on government action before we get any.
I notice that we are already committed to 1.4 degrees C of eventual temperature rise, which is a long way already toward the 2 degrees C which some regard as a point we really should not exceed …
I note also that CO2 concentrations are not just increasing but accelerating upward in uncontrolled manner .
The lowest graph above indicates that to bring CO2 down again under control that we simply must bring CO2 emissions below pre-industrial levels , something with absolutely massive implications for our way of thinking , our work, and our lifestyle, changes which are being currently almost completely ignored by our governments.
So how few years do we have to act to prevent eventual disaster of 2 degree C temperature rise and what is involved if we do change our ways and act to save ourselves?
Is it even possible in any realistic plan for the whole world to change so much in those few years when in fact we are still going the wrong way by increasing CO2 output, not reducing it ?
What then are the real targets we need to guarantee reaching and time frame involved to stop catastrophic human deaths beyond say ten times the current unacceptable levels ? … since it would seem that we will not treat the matter as urgent and initiate emergency changes until we know in a format many can understand, that is before the results are felt painfully by most and have become irreversible .
The problem it seems is that we have walked blindly into a trap , since global warming continues long after we act to slow it… but we are still looking at the comfort of our lifestyle now rather than the misery and death we are buying for our future by inaction, the real inevitable consequences of inaction are not being seen by most, nor the rate of response required for acceptable mitigation of human loss of life.
One study (below) indicates that we are already committed to a further 0.5 degrees C rise even if we were to control emissions to year 2000 levels , which we are not doing … is the implication then that we are already committed, by inaction, political inertia, to future catastrophic damage to our home planet’s ability to support human life… the death count simply determined (exponentially with time , up to human extinction) by our level of inaction in the next few years. Why would an intelligent species not minimise the long-term risk of catastrophe to itself ?
http://www.ifm-geomar.de/fileadmin/personal/fb1/p-oz/mfrank/Bard_and_Frank_2006.pdf
Death rate from global warming is already recorded at WHO and already unacceptable ,but what is not stated is how many future deaths are already inevitable as a result of damage already done which we know has worsening consequences for centuries to come :-
http://en.wikipedia.org/wiki/Global_warming
September 6th, 2008 at 6:47 pm
Thanks for your comments.
The graph you mention above highlights not simply that emissions must reduce below pre-industrial levels, but that we must draw down carbon from our atmosphere.
As also mentioned above, this post is an extract from the Transition Timeline project which is indeed examining exactly how we can go about addressing those massive changes in our cultural stories and practices to which you refer. The project will be published in late 2008 (as soon as I can manage!)
The Transition Towns movement is a rapidly growing force in addressing the imperative to keep temperatures at a level to which life on Earth can adapt. As you highlight our Government is not currently addressing the same question, and only massive popular demand for a future can change this.
To my mind, the barrier to such overwhelming demand is not the lack of a desire for a future for ourselves and our children, but rather a lack of widespread understanding of the emergency we face. It is to the amelioration of this that this post is dedicated.
As I outlined in a previous post, I do believe that it is still realistic to change our perspective sufficiently to enable that future, and to answer your question directly my present best assessment would be that we need to stabilise global emissions levels by 2012 and start rapid reductions from there in order to achieve this (though climate change is not our only challenge).
It is perhaps relevant to note here that the physical limits to fossil fuel supply rates mean that continuing the current rate of increase in anthropogenic emissions is impossible anyway (see here and here) - the current cultural paradigm is meeting its limits today, in ways that are being experienced in present, direct and immediate ways. The tussle we must engage with is in providing the stories that will succeed it. In this humanity faces our last chance to choose the consequences we desire, for ourselves and all of Nature.
“I am always surprised when people get depressed rather than energised to do something. It’s not too late to stabilise climate… I am not about to give up” – NASA’s Dr. James Hansen (Sept 2007)
October 23rd, 2008 at 7:50 am
[...] here’s an explanation of climate change written as a 101/glossary of the information you need to interrogate [...]
October 27th, 2008 at 12:37 am
You say: Since a hotter planet loses more energy into space, the natural system tends to move towards thermal equilibrium.
How then do you explain the fact that prior to the use of fossil fuels by humans, the earth’s temperature naturally fluctuated considerably?
October 27th, 2008 at 11:13 am
Hi DocBud - I’m simply highlighting that since space is cold and the temperature inside our atmosphere is relatively hot, heat energy dissipates into space. Clearly if the temperature inside the atmosphere increases then the difference between that and the (stable) temperature of space also increases, leading to a greater rate of temperature dissipation (AKA radiative forcing). So there is a negative forcing (a damping forcing) here - if temperature increases (or decreases, for that matter) this effect tends to bring it slowly back towards a new equilibrium point.
As you say (and as I explain in the rest of the article) this negative forcing is far from the only effect on global temperatures, but it is important to understand it before exploring the other factors.
In direct answer to your question, there are a great many factors that affect global temperatures (see Milankovich cycles). Indeed, the evidence suggests that we would actually be heading into a period of slow global cooling now if not for the effects of human emissions, which have overwhelmed the other factors and become the overriding driver of the current dramatic temperature increases.
October 27th, 2008 at 1:54 pm
Despite the ongoing increase in anthropogenic CO2 (you don’t discuss the far greater quantities of natural CO2 emissions or water vapour), we are entering a period of global cooling (the planet has not warmed since 1998 and has shown recent cooling, we are back to where we were in the 1980s). The theory is being falsified by real world data and yet people still dishonestly talk of dramatic or unprecedented temperature increases. There is nothing dramatic or unprecedented about the current climate which begs the question: what is your agenda? It sure isn’t saving humans or any other species, we and they will all do fine in a warm, healthy, productive planet and if the earth cools and becomes less hospitable, I’m sure most of us will adapt.
October 27th, 2008 at 3:08 pm
Sigh. These days I generally don’t respond to such disinformation, but as others might be misled by it here…
Natural carbon and water cycles have always existed - it is our moving of inactive carbon (stored underground as coal etc.) into the atmosphere that has moved us away from equilibrium.
The Earth is not cooling and the temperature increases certainly are dramatic, especially considering that the systemic delays explained above highlight that they are only the beginnings of what we have committed ourselves to.
Probably the two most robust global records of measured temperature are:
Climatic Research Unit direct surface temperature analysis
NASA GISS direct surface temperature analysis
To call people ‘dishonest’ for quoting the evidence on this matter is deeply irresponsible.
October 28th, 2008 at 9:43 am
[...] kicked into gear. It seems that her article (or possibly some comments from Doc Bud on a post from Dark Optimism) were picked up by Australia’s Herald Sun columnist Andrew Bolt, and an Englishman’s [...]
October 31st, 2008 at 1:51 pm
“Sigh. These days I generally don’t respond to such disinformation”
You being a climatologist one can understand why, but your first link clearly shows a recent cooling trend, even if using exaggerated y-axes and long time scales tends to obscure the reality of the last ten years.
November 5th, 2008 at 8:25 pm
There are always many short term effects and variations - these change neither the extraordinarily clear present trend nor the improving understanding of the reasons for it. Looking at the same graph, your argument is akin to claiming we were entering a period of global warming based purely on the increase around 1880.
For a better understanding of the nature of climate prediction and the reasons why basing predictions on a few cool or hot years is inappropriate take a look at this excellent presentation from Oxford University.
November 9th, 2008 at 11:15 pm
Shaun, I appreciate your scientific insights (There are always many short term effects and variations).
I’m a bit confused as to what constitutes short term and long term. It seems ten years of static to cooling temperatures (although someone emphatically stated “The Earth is not cooling and the temperature increases certainly are dramatic”) is too short a period to suggest that temperatures do not correlate to human emissions of carbon whereas a little over 20 years is long enough to determine that there is a correlation and in response we need to commit global economic suicide, condemn the world’s poor to greater poverty and restrict people’s freedoms to live how they choose to live.
I did follow the link to the allegedly “excellent presentation from Oxford University” which probably helps to illustrate why Oxford is such a poor cousin to Cambridge these days. It starts and finishes with the assumption that human emissions are a significant driver of climate change without, as no individual or organisation does, offering a shred of proof.
November 10th, 2008 at 3:51 am
As already explained, our understanding of anthropogenic climate change is not limited to an observation of temperature change. We understand ever better how and why our actions to date will tend to increase radiative forcing over the long-term. Indeed, no-one has suggested a remotely scientifically plausible hypothesis as to why dramatically increasing the concentrations of known greenhouse gases in the atmosphere would fail to do so.
There is also an improving understanding of the short-term cycles which are currently pushing against this long-term warming forcing and which are expected to cycle to push with it shortly after. Most predictions are that the warming will slow over the coming years (great news in theory, terrible news if it means that many will mistakenly believe the problem has gone away) before accelerating to catch up with the long-term forcing.
I am writing in shorthand because I am busy and, unfortunately, because I get the feeling you already know all this. It troubles me that you have the graph in front of you with the temperatures from ten years ago and the (substantially higher) current temperatures yet you still claim “ten years of static to cooling temperatures”.
November 10th, 2008 at 2:20 pm
“We understand ever better how and why our actions to date will tend to increase radiative forcing over the long-term.” Easy to type, very difficult to verify, perhaps you might like to try.
“Indeed, no-one has suggested a remotely scientifically plausible hypothesis as to why dramatically increasing the concentrations of known greenhouse gases in the atmosphere would fail to do so.” What a load of old tripe, the actual science says quite clearly that modest increases in minor greenhouse gases cannot have the dramatic consequences claimed. Only when we step into the virtual reality of numerical models and imagined climate forcings do things become scary.
“There is also an improving understanding of the short-term cycles which are currently pushing against this long-term warming forcing and which are expected to cycle to push with it shortly after. Most predictions are that the warming will slow over the coming years (great news in theory, terrible news if it means that many will mistakenly believe the problem has gone away) before accelerating to catch up with the long-term forcing.”
Before the recent non-warming there wasn’t any such understanding and all predictions were of continued warming (Hansen’s and the IPCC’s most certainly were), but to keep the scary pot boiling the alarmists had to change their story for fear people might see the emperor has no clothes.
“It troubles me that you have the graph in front of you with the temperatures from ten years ago and the (substantially higher) current temperatures” You are joking, of course.
http://davidsmith1.files.wordpress.com/2008/06/0603084.jpg
But you are not alone, even your heroes tell porkies.
http://wattsupwiththat.com/2008/11/07/truly-inconvenient-truths-about-climate-change-being-ignored-ipccs-pachauri-says-warming-is-taking-place-at-a-much-faster-rate/
“I am writing in shorthand because I am busy” The last vestige of the person with no argument.
November 10th, 2008 at 4:47 pm
You will find that I have actually often been critical of Pachauri and the IPCC, but his statement here is accurate. As just outlined the short-term noise does not alter the fact that the warming trend is indeed developing at a faster rate than before. As explained in that Oxford presentation you mocked, a few cooler years do not change this trend. If you have no evidence for any alternative hypothesis other than an unexplained observation of temperature then I suggest you cease mudslinging and pay attention to the scientific consensus which does explain this observation.
The rest of your comment appears to consist of unsubstantiated falsehoods. If the IPCC’s findings do not qualify as “actual science” I have no idea what you are suggesting does.
November 13th, 2008 at 11:56 pm
Sorry about the delay, I’ve been working on site and my client’s server blocks access to pretty much any site that is not news, science or engineering based.
The simple fact is that the scientific consensus (a meaningless consept in science as Einstein observed) does not explain climate change. If you can point to anything in IPCC AR4 or previous reports that constitutes proof, as opposed to vague assertions, of the validity of the hypothesis that human CO2 emissions are primarily responsible for recent climate change, please do. The IPCC is a prime example of pre-conceptual science, the outcome is predetermined, the evidence is then cherry picked to suit the desired outcome. With the IPCC, the science is made to suit the Summary for Policymakers, not the other way round as is the case for most scientific endeavours (although we do see similar types of behaviour in other public policy areas such as obesity and passive smoking). The IPCC’s science amounts to, here is evidence that the climate changes, we believe that this is mostly caused by human CO2 emissions and that if humans carry on emitting this will happen and this is what we think will be the impacts.
Climate science is still a poorly understood field of research and to claim that anything we do not yet understand must be due to human CO2 emissions is not good science. In essence that is what is happening, human CO2 emissions alone are not sufficient to explain current climate change so the computer models have to be programmed to amplify the effect, the justification being that there are positive feedbacks. Any hypothesis is falsified when it does not match reality, the human influenced global warming hypothesis has been falsified by the absence of a meaningful correlation between atmospheric CO2 and global temperatures. (Incidently, I did notice that you made no attempt to justify why 10 years is a short term but a little over 20 years is sufficiently long to conclude that human CO2 emissions are the significant driver of recent climate change).
The list of climate drivers or potential drivers (including CO2, mostly natural, miniscule amount human) is very long and their relative importance poorly understood. These include: cosmic ray flux, solar magnetic cycles, sunspot cycles, cosmic dust, the shape of the Earth’s orbit, ‘wobbles’ of the Earth upon its axis, changing rotational velocity of the Earth’s core, changes in the Earth’s magnetic field, tectonic movements of the Earth, volcanic eruptions, changes in the circulation patterns of the oceans, changes in ocean salinity and chemistry, changes in atmospheric water vapour, clouds and cloudiness, changing albedo of Earth through landscape change, natural and human, changes in vegetation and fires (natural and human). Despite these and other factors, known and probably unknown, we are supposed to harm the global economy, and hit the poorest hardest, by restricting the emissions of miniscule quantities of a minor trace gas whose actual influence is likely to be equally miniscule.
The primary reason why it is not good science to ascribe current climate change primarily to humans is that, as any geologist will tell you, there is nothing unprecedened about what is happening now.
http://www.globalresearch.ca/index.php?context=va&aid=10783