No, it is not true that it invalidates climate change claims, or the need to limit further increases in CO2 concentration specifically as it relates to warming (we can ignore stuff like ocean acidification for this question to keep from wandering too far abroad). First of all, let's look at the source. Is the person an objective scientist, making claims and analysis from professional expertise, or from actual research? No.
The source cited in the question is a professional, paid climate science denier. He's best known for creating and hyping faux controversies by misstating facts relating to emails between climatologists and for publishing scientifically invalid misinterpretations of NOAA data. He is paid by the Heartland Institute, which is a phony industry "think tank" paid for by industrial polluters. In this case he appears to be doing the same type of spin on IPCC findings.
SourceWatch: Anthony Watts
But being a paid shill does not, in and of itself, mean he's wrong. Having a history of getting the science wrong does not mean, in and of itself, that he's getting it wrong here. It would lead one to be less likely to give the benefit of the doubt, but if he's wrong, there should be a relatively straightforward explanation of why and how he is wrong.
Here's an explanation of what's wrong with the article that Watts is citing and quoting:
Ed Hoskins' pseudo-science claptrap brings out all the nutters at WUWT
Before going any further, it will pay to go back to the logarithmic relationship between atmospheric CO2 and global surface temperature. In short, what it means is that for every doubling of atmospheric CO2, the surface temperature will rise by the same fixed amount, between 1.5°C and 4.5°C, probably by around 3°C over the medium term (centuries). Many deniers think it means that for every doubling of CO2, temperature will rise much less, but that's wrong. It will go up by roughly the same amount when CO2 doubles (at foreseeable levels). The typical science denier doesn't do maths.
Ed's article is all over the place like a dog's breakfast, but his main very wrong message is that CO2 can rise to 1000 ppm without any bad consequences. That's just not so.
Ed's argument is that because the effect of CO2 on temperature is logarithmic, it will only have a tiny impact as it increases. The first part is true. The relationship is logarithmic. However the second part is relative. What might seem a "tiny impact" for, say, a diurnal temperature variation would have an enormous impact if it were an increase in average surface temperature over the entire world. From our perspective, the impact on climate and ocean acidification and rising sea levels will be hugely damaging as more and more CO2 is emitted.
A rise in atmospheric CO2 from pre-industrial times (say 280 ppm) to 1000 ppm would mean an increase of 3.5 times. That would mean a rise in global surface temperature of somewhere between around 2.8°C and 8°C degrees or more.
According to an analysis by Steven C. Sherwood and Matthew Huber, a rise of 7°C in average surface temperature would result in a wet bulb temperature of more than 35°C in much of the world, which would stretch our physiological limits beyond tolerance. We wouldn't be able to maintain our core body temperature and would overheat. People die from heat stroke every year as it is, without a wet bulb temperature of 35°C. We cannot survive temperatures above a certain level and humidity. We can't sweat it out.
I wondered how Ed Hoskins came up with his magical thinking. He went back to the third assessment report of the IPCC for some reason, back thirteen years to 2001. He wrote:
quotes the same passages as the OP here
He thinks that something "is well disguised". Well, it looks as if it is so "well-disguised" that Ed himself cannot understand it. Thing is, it doesn't "radically diminish". He wrote about the IPCC publishing views about the effect of CO2 up to 1000 ppm, but doesn't show it. Instead he shows charts from denier Steve Milloy who admits that what he writes is junk science. (That's the name of his blog.)
So does the IPCC publish views about the effect of CO2 up to 1000 ppm? There were only four mentions of the words "1000 ppm" in the latest IPCC report, and three of them referred to the same period in Earth's history, the Early Eocene Climatic Optimum. (The other was a reference to stomata.) Here are two of them:
During the Early Eocene (52 to 48 million years ago), atmospheric CO2 concentration exceeded ~1000 ppm when global mean surface temperature was 9°C to 14°C higher (medium confidence) than for pre-industrial conditions. [IPCC AR5 WG1:TS.2.8 Changes in Carbon and Other Biogeochemical Cycles]
The EECO [Early Eocence Climatic Optimum] represents the last time atmospheric CO2 concentrations may have reached a level of ~1000 ppm (Section 184.108.40.206). There were no substantial polar ice sheets, and oceanic and continental configurations, vegetation type and distribution were significantly different from today. Whereas simulated SAT are in reasonable agreement with reconstructions (Huber and Caballero, 2011; Lunt et al., 2012) (Box 5.1, Figure 1d), there are still significant discrepancies between simulated and reconstructed mean annual SST, which are reduced if seasonal biases in some of the marine proxies are considered for the high-latitude sites (Hollis et al., 2012; Lunt et al., 2012). Medium confidence is placed on the reconstructed global mean surface temperature anomaly estimate of 8°C–14°C. [IPCC AR5 WG1: 5.3.1 High CO2 Worlds and Temperature]
In other words, the last time when the atmospheric concentration was 1000 ppm or more was 52 to 48 million years ago and the global mean surface temperature was 9°C to 14°C higher than it was in pre-industrial times in this modern era. Does that seem like a tiny effect to you? Does it strike you as inconsequential that vast areas of land would become uninhabitable because the wet bulb temperature would be intolerable for humans?
In a nutshell, being logarithmic doesn't magically cap the upper bound. If we more than double the atmospheric CO2 (currently around 400 ppm) to 1000 ppm, we will see more than the amount of warming stated in the question. And that ignores a lot of other factors, as well, but since the basic concept is wrong, we don't need to delve into that.