I have to disagree with the definition of "climate change" in the opening post Climate Change is looking at long term data and seeing if there's a change in the overall climate. Unfortunately, you have to look at very long periods to get any significant data, because there are some long term oscillations in the Earth's climate and you need to take those into account. If you take too small a period of time, you can make some mistaken assumptions.

For example, here in Melbourne, we recently went through an extended (13+years) of lower than "average" rainfall. There was talk about this being the new norm (ie. the new climate) because it was extending for so long. However, a couple of years ago the rain returned and we're now getting average to above average rain again. The suggestion that our climate had been permanently changed appears to be erroneous.

Generally, for climate we're talking about 30+ year periods. A graph of global average temperature over the last 120 years shows what appears to be a steady upward trend, with an imposed 60 year oscillation (30 years above the trend, 30 years below, 30 years ago, etc.). This has been going on since the time of the Little Ice Age (LIA) back in the 1700/1800s.

There is no doubt that the climate has changed since that time. The issue is how much humans have been influencing the change. There's no real significant doubt that we can affect micro-climates (the Urban Heat Island effect is a good example of this, with cities generally being hotter than the surrounding countryside). Globally, however, our impact is much reduced, given that 70% of the Earth's surface is ocean and not populated.

Current theory states that the increase in CO2 levels in the atmosphere will result in rising temperatures. The basic science says that there will be approximately 1C increase in global temperatures for each doubling of CO2 levels in the atmosphere. It's what happens next that there's a lot of uncertainty. Climate models suggest that that 1C increase is then amplified via various feedback mechanisms to a between 1.5 and 4.5C increase.

Unfortunately, the climate models are currently not tracking particularly well when compared to current global temperatures. We don't know if that is due to natural variability or if the climate models have got problems. For example, the climate models don't deal with clouds very well, and clouds are a major feedback item. Indeed, the question of whether clouds are a positive or negative feedback (clouds at night keep temperatures higher, but clouds in the day keep temperatures lower) is still in doubt. The degree of feedback is also unknown.

Similarly, ENSO is a natural phenomenon in the Pacific ocean that has a major impact on the climate throughout the pacific region. We're talking something like 30% of the Earth's surface. Climate models can't currently model the ENSO. There are questions as to whether they need to (it has been proposed that ENSO is ultimately neutral, that the positive and negative swings cancel out in the long term), but that's not proven. If ENSO is a climate driver, rather than a climate reaction, then the climate models are not taking that into account.

What is going to happen long term? Who knows....

Should we be doing something about it, just in case? I believe we should. However, I believe we should be doing things that are shown to have a real impact. At the moment most of the emphasis is on reducing CO2 levels, but the correlation between CO2 levels and global temperatures is getting poorer each year. That may change this year, but if it doesn't, then trying address climate change through just the one mechanism may be fatally flawed.

This is a big topic with lots of fine detail. Sometime the fine detail is largely irrelevant (such as trying to map extreme weather to climate change -- even the IPCC has rejected that there is any evidence for a link), but sometimes it's not (such as trying to work out the real climate sensitivity -- that is, how much temperatures will rise as CO2 levels rise).

Sadly, all too often all we hear are sound-bites on the subject

I agree with you that there are multiple climate zones around the world, but the term "climate change" is generally considered to be long term change in a climate, not short term. I can, however, concede that this can correspond to changes in each climate zone (and it really should be for all analysis, even if that's not the common usage).

I agreed in my original post that we've changed climates locally, giving the example of the urban heat island effect around cities. However, our impact on climates outside of our civilisations is more problematic to measure. In particular, our impact on the middle of the oceans.

You've included rising sea levels as part of your comment. Sea levels have been rising for thousands of years. If you check out sea level rises on a geological scale, recent sea level rises are insignificant. Yes, the sea level does appear to be rising (though one prominent ocean scientist disagrees, claiming that he was told that the satellite data was adjusted to produce an increase because it wasn't there in the raw data -- I don't have the expertise to comment on that, just to note it). However, assigning human activity as being responsible for that change is not an easy task. It is certainly possible that human activity has resulted in an increase in ocean temperature and hence sea level rise through thermal expansion, but that possibility is a long way from being proven.

I also agreed on the fact that CO2 is a greenhouse gas and each doubling would result in a rise of 1C (technically 1K) in global temperatures. That's the scientific fact as verified in scientific experiments. It is the degree of amplification through feedbacks that has not been demonstrated with any certainty. Climate models predict a range of 1.5 to 4.5C, but observational analysis currently only supports the lower end of that range (with some analysis giving figures below 1.0).

Please do not use Venus as an example, or I will use Mars as a counter example, as it has an atmosphere with a much higher concentration of CO2 than Earth's, but it doesn't show the effect that is supposed to happen from high CO2 levels Realistically, though, neither is comparable to Earth because neither possess the number one greenhouse gas in any reasonable quantity in their atmosphere. That greenhouse gas is... water.

Water (and hence clouds) is the wild card in the climate analysis stakes. All feedback theories rely on water in the atmosphere to increase temperature gains. However, it depends on the state of that water. If the water is vapour, it is a greenhouse gas that increases the effect of a temperature increase. If, however, the water is condensed into clouds, it will lower maximum temperatures (during the day -- cloudy days are cooler than sunny days) and raise minimum temperatures (cloudy nights are warmer than cloudless nights). Working out how temperature changes will affect cloud coverage, and hence the feedback effect, is an unsolved problem in climate science. Cliimate models don't know what will happen -- they simply include a factor for clouds with no certainty that the way they treat clouds corresponds to reality.

For the period leading up to the turn of the century, there was a strong correlation between CO2 increases and global temperature increases (to the limit of being able to calculate it -- there are a lot of issues with record qualities in thermometer readings over historical decades). However, since the turn of the century, the correlation is close to zero. CO2 levels, as measured by the Mauna Loa observatory, has been going up steadily. Global temperatures have not. This has not been satisfactorily explained in the scientific literature. There have been some suggestions (such as trying to remove the ENSO signature from the temperature record), but they all have flaws (such as if the ENSO signature is removed, then the correlation no longer corresponds to something real -- it's correlating to an artificial number, rather than the actual global temperature). There have been a number of proposals, but nothing that's being hailed as "the explanation".

One proposal has been that the "missing heat" is going into the oceans. Unfortunately, that isn't confirmed by the Argos or Tao bouys. A re-analysis of the ocean depth data suggests that the deep ocean is increasing in temperature, but it's uncertain. The amount of temperature increase is a small fraction of a degree, and the error range for the measurements more than swamp that figure.

Alternatives to CO2 levels being the driver of the climate? There are a few.

The first is simply that current changes are within the range of natural variability. This proposal relies on factors such as the higher temperatures during medieval and Roman eras (not proven, but not disproven, either), and that the temperature has been going up well before CO2 levels had changed significantly (ie. since the Little Ice Age). Is this proposal proven? No. It hasn't been disproven, either, because we're unable to fully explain all factors that affect the climate. This explanation, does, however, satisfactorily explain sea level rises, because historically current rates are actually quite low when compared to geological records.

The second is a change in cloud cover. It has been estimated that it only requires a change of global cloud cover of a few percent to explain all warming observed to date. Unfortunately, we have no good historical record of global cloud coverage. We only really have records for the satellite era, which is too short as it doesn't even cover one 60 year climate cycle.

The third is changes in Sun output. This relies on the concept of a Grand Maximum in sunspot numbers (sunspot number correlate with Total Solar Irradiance - TSI). However, certain solar scientists believe that current sunspot numbers are inflated when compared to how they were counted historically. When taking that into account, there is no Grand Maximum and hence this theory can be discounted.

Regarding the Precautionary Principle, I support it. As I said in my earlier post, I believe we should take actions that have been shown to have an effect. Since changes in CO2 levels correlate poorly with global temperatures for the last 15-20 years, looking purely at CO2 is probably short sighted and not cost effective. Humans have other impacts on the environment besides CO2, and we should also be looking at those items to work out what we can do.

I pointed out in my earlier posts "we don't fully understand climate mechanisms and we cannot currently predict what changes will occur and when" but "Global warming is about averages".

 

1. the planet's average temperature has been increasing over the last 100 years [if you dispute this then share your data]

2. this coincides with a rapid increase in human population [if you dispute this then share your data]

3. this coincides with a rapid increase in human greenhouse gas "production" [if you dispute this then share your data]

4. greenhouse gases cause warming [this seems to be agreed]

 

To put it simply

- we see an effect: warming

- we have a cause for that warming, not the only cause, but a material cause

 

As for your request "Please do not use Venus as an example, or I will use Mars as a counter example, as it has an atmosphere with a much higher concentration of CO2 than Earth's" , well I'm very happy to use Venus as an example because Earth and Venus both have thick atmospheres (14.69 psi and 1,330 psi respectively). Mars's atmosphere, on the other hand, is  so thin and tenuous (0.087 psi) that it's CO2 atmosphere - about 0.6% of Earth's mean sea level pressure - has no measurable warming capacity

 

Don't confuse correlation with causation.

Firstly, I agree with all four points. The problem is that the numbers don't add up.

Temperatures have been rising for longer than the period where CO2 is generally accepted to have increased significantly. It's the fine detail such as this where things matter.

Temperatures have been increasing since around the 1850s. The CO2 increases are generally considered to be only to become significant from the 1950s, and to only have a significant impact on the temperature from the 1970s.

The question is what drove the temperature increase from the 1850s to the 1970s, and why isn't it still a significant factor from the 1970s to now? There have been numerous papers proposing answers, but there are no generally accepted answers, only theories.

The next question is if CO2 is the primary driver of climate change (there are papers that claim it is responsible for most of the temperature increase since the 80s), then why are global temperatures roughly stagnate for the last 15-20 years while CO2 has been increasing at a steady rate?

There is a paper out there that claimed we would need a minimum of seventeen years of data to be able to identify the human impact on the climate (prior to that paper, it was generally considered to be ten years). According to one of the satellite-based temperature records (the major temperature records are HADCRUT3, HADCRUT4, GISS, RSS and UAH, with the last two being satellite records -- the new BEST temperature record is for land only at the moment), we've already at seventeen years with no significant increase in global temperature. If that's confirmed by the other records in the future, then according to that paper, the link between CO2 and temperature would appear to be disproved. The other three global temperature records do not show a zero trend for the last seventeen years, but all show a significantly reduced rate of increase compared to the rate for the twenty or more years before that period.

If global temperatures had continued to rise since the turn of the century, then that would be a strong argument to saying that CO2 is a major driver of world-wide temperatures. However, all of the temperature records have shown a significantly decreased trend over the last two decades compared to the two decades before. That doesn't disprove the CO2 driver theory (because there is still natural variation), but it does show we don't know enough to be confident that CO2 is a major driver.

CO2 has an impact, as we've both agreed. What is not known is how much of an impact it has, as per the climate sensitivity ranges I mentioned in my previous post. It is quite possible that due to negative feedback, it has an insignificant impact. The jury is still out on that. Model-based estimates of climate sensitivity indicate it isn't non-significant, but observation-based estimates include ranges that are insignificant.

Most of the future disaster predictions come from the computer models used to project (not predict) future climates. Unfortunately, those models continue to fail to match up with observations in many areas. That could be because of natural variation, but given the paper I mentioned earlier (Santer. et. al), the last 17-20 years of temperature data implies that the climate models run hot when compared to reality. That means we have to take their projections with a huge grain of salt.

I would also like to thank you. As you noted in your original post, this is a subject that tends to get very emotional. It's really nice to discuss the subject in a calm and rational way

PS: Venus doesn't have the water vapour in its atmosphere that Earth does, and it is widely acknowledged that water vapour is the single most significant greenhouse gas. Also, Venus has a much higher atmospheric pressure than Earth, and temperature is correlated with pressure. When the numbers are crunched, Venus does not provide the evidence you think it does