The Intergovernmental Panel on Climate Change said this in its summary (for policymakers) of its latest major report:
It is extremely likely (>95% confidence) that human influence has been the dominant cause of the observed warming since the mid-20th century.
This report, the Fifth Assessment Report, was preceded by one whose summary said:
Most of the observed increase in global average temperatures since the mid-20th century is very likely (>90% confidence) due to the observed increase in anthropogenic greenhouse gases.
Both reports also discuss the total amount of warming observed since the nineteenth century, but as these quotes show, a key issue in the global warming discussion is what portion of the warming since 1950 has been caused by human influences on the planet's climate. Some people would say 0%. Others would say what the IPCC has said in the past, that it is 50+%. Others would say it is over 100%. I think that last one is nonsensical, but that's a topic for another post.
What's most important, however, is there's a group of people who say the answer is influenced by the "fact" human adjustments to the temperature record have artificially exaggerated the amount of warming that has actually occurred. They may think the exaggeration is slight, or they might think it is so large as to completely fabricate the apparent warming trend. Imagine telling them things like:
— Robert Way (@LabradorIce) November 28, 2015
Wouldn't that seem a little off? If a person's concern is adjustments to data which exaggerate warming, would you think telling them what adjustments do since ~1900 would address their concerns in a clear and convincing manner? I don't think so. I think a lot of people might well look at the IPCC attribution statements and focus on the period after 1950, where the human contribution is said to be the major factor. That's why I told that user, Robert Way:
— Brandon S? (@Corpus_no_Logos) November 29, 2015
But it gets worse than that tweet indicates. 1950 is commonly given as the breakpoint where the human effect on the planet's temperatures becomes discernible, but that isn't when humans first started having an effect. I'll use a post from Judith Curry as an example of people discussing the issue this causes. She writes:
The IPCC AR5 doesn’t have much to say about the early 20th century warming or the grand hiatus (Section 10.7.1.1):
"The AR4 concluded that ‘A substantial fraction of the reconstructed Northern Hemisphere inter-decadal temperature variability of the seven centuries prior to 1950 is very likely attributable to natural external forcing’. The literature since the AR4, and the availability of more simulations of the last millennium with more complete forcing, including solar, volcanic and greenhouse gas influences, and generally also land use change and orbital forcing) and more sophisticated models, to a much larger extent coupled climate or coupled earth system models, some of them with interactive carbon cycle, strengthens these conclusions."
So, what does all this mean for IPCC’s ‘extremely likely’ attribution statement for the warming since 1950?
1. There have been large magnitude variations in global/hemispheric climate on timescales of 30 years, which is the same duration as the late 20th century warming. The IPCC does not have convincing explanations for previous 30 year periods in the 20th century, notably the warming 1910-1940 and the grand hiatus 1940-1975.
2. There is a secular warming trend at least since 1800 (and possibly as long as 400 years), that cannot be explained by CO2, and is only partly explained by volcanic eruptions.
The combination of these two points substantially reduces the confidence that we should place in attribution statements of warming since 1950. Getting the ‘right’ answer (i.e. explaining the warming from 1970-2000) for the wrong reason (i.e. CO2) simply has not been eliminated as a serious consideration, and hence an ‘extremely likely’ conclusion is highly inappropriate.
For a simplified argument, if the amount of warming observed prior to 1950 (before humans had a dominant role) was comparable to the amount of warming observed after 1950, that casts some level of doubt on the idea humans are the dominant cause for the warming since 1950.
Simple, right? The more warming there was prior to 1950, the more it suggests non-anthropogenic causes. The more warming there is after 1950, the more it suggests anthropogenic causes. That means adding warming to the recent portions of the temperature record would strengthen the argument for large amounts of AGW while adding warming the earlier portions of the temperature record would strengthen the argument for small amounts of AGW.
There is nothing complicated or complex about this. Judith Curry has written about this idea tons of times, and hundreds of other people have done the same. That's why I told Robert Way:
— Brandon S? (@Corpus_no_Logos) November 29, 2015
He ignored me. I wouldn't think much of that except he still tweets the same argument, ignoring the simple point I explained to him, months later like this tweet a few days ago:
— Robert Way (@LabradorIce) January 26, 2016
He doesn't actually refer to the whole century this time, but it's easy to see that's what he's talking about. Here is a convenient post to see for yourself. It's by Victor Venema, who has been the topic of a couple posts recently due to his gross misrepresentations of people's arguments which he uses to paint global warming skeptics as insane. The post I'm referring to now isn't much better, but it is convenient due to showing the effect adjustments have on temperatures. As he explains:
The first plot is for the land surface temperature from climate stations. The data is from the Global Historical Climate Dataset (GHCNv3) of NOAA (USA). Their method to remove non-climatic effects (homogenization) is well validated and recommended by the homogenization community.
They adjust the trend upwards. In the raw data the trend is 0.6°C per century since 1880 while after removal of non-climatic effects it becomes 0.8°C per century. See the graph below. But it is far from changing a cooling trend into strong warming. (A small part of the GHCNv3 raw data was already homogenized before they received it, but this will not change the story much.)
If you only look at temperatures over land, ignoring the majority of the surface of the Earth which is covered by water, this is the effect adjustments have (on this particular data set):
The adjustments made to land-only data increase the total amount of warming we find. People might say, "Ah-hah!" as though that's proof of some nefarious intent, but look at that chart. The raw and adjusted data match each other very well in present times, only diverging as one goes further back in the past (caused in no small part due to arbitrary aspects of the methodology used).
That means the total amount of warming may be less in the raw land-only data, but the effect is to actually increase the apparent effect of natural variability. As one moves to more recent portions of the record, when human influences are thought to be more dominant, the adjustments have little to no effect. It's only in the past when natural variability is thought to have a greater role (percentage-wise) that these adjustments increase the warming in the record. That means these adjustments increase the apparent effect of natural variability.
Similarly, when Victor Venema says:
Not many people know, however, that the sea surface temperature trend is adjusted downward. These downward adjustments happen to be about the same size, but go into the other direction.
Being land creatures people do not always realise how big the ocean is, but 71% of the Earth is ocean. Thus if you combine these two temperature signals taking the area of the land and the ocean into account you get the result below. The net effect of the adjustments is a reduction of global warming.
He implies one should think the fact these adjustments decrease the total amount of warming means it is foolish to think climate scientists have made them to exaggerate global warming. Only, look at the chart he shows:
Again, what we see is temperatures in more recent times are largely unaffected. It is primarily the past which has been adjusted. So while it is true these adjustments shrank the total amount of warming, they did so primarily in the period where that warming would be more likely to be attributed to natural variability. In the period where warming would be (almost) solely attributed to human influences, the adjustments have little effect, but actually increase the total amount of warming.
That anyone would offer this as a counterargument to global warming skepticism is remarkable. It is true this argument effectively proves adjustments to the data do not manufacture global warming out of thin air. It is also true this argument effectively proves adjustments to the data do not exaggerate global warming as a whole. But in doing so, this argument makes something of a case that adjustments to the data exaggerate anthropogenic global warming.
And really, that's what we all care about. The global warming debate, by far and large, is not about whether the planet has warmed since ~1850. The global warming debate is about how much humans have contributed to that warming, and how much they will continue to do so in the future. Pointing out adjustments to the data lessen the apparent role of non-human factors can only help fuel global warming skepticism.
The strangest part of all this, however, is that people like Robert Way and Victor Venema don't realize this. Zeke Hausfather, the person who actually made the graphs used in Venema's post, doesn't seem to realize it either even though he works on one of the temperature record projects (BEST). Many climate scientists who repeat this argument seem to fall into the same trap as well. I'm not going to make a long list of names because this post isn't about calling out specific individuals.
It's just my way of saying, "What the hell, guys?"