Thompson’s Dunde ice core is an extremely important proxy in multiproxy studies. There has been an increase in dO18 levels in the 20th century. Whether this is a proxy for temperature is not at all obvious on physical grounds. The relationship between dO18 and temperature in monsoon ice caps is opposite to that of polar regions ” the lowest dO18 occurs in summer rather than in winter. Thus, a greater proportion of summer precipitation on the ice cap would cause lower dO18. The supposed connection between increased 20th century dO18 and temperature is entirely statistical. Thompson et al  says:
The 5-year running means of dO18 from the Dunde Ice Cap are compared in Figure 5 with 5-year running means of Northern Hemisphere temperatures (Hansen and Lebedeff, 1987). For the period from 1895 to 1985, the correlation coefficient r is 0.5 (significant at the 99.9% level). This correlation suggests that the Dunde Ice Cap dO18 should serve as a good proxy for larger-scale temperature variations.
I’ve been meaning to check this for a while as it only takes a few minutes to do.
I checked Dunde against CRU NH temperature (I have this, not Hansen and Lebedeff, but the result should not depend on temperature data set). Of the various Dunde version in circulation, I used the Dunde version archived at MBH98, which visually looks like the version in Figure 5 of Thomspon et al  and is available on an annual basis.
The correlation for the period 1895-1985 – the period cited by Thompson was 0.48, almost exactly the same as the reported coefficient of 0.5. Since Thompson is on the Hockey Team, you have to ask yourself why he only did the correlation from 1895 on. Any bets on what the correlation was for 1851-1895? Minus 0.36.
Thompson, L. et al. "Recent warming": ice core evidence from tropical ice cores with emphasis on central Asia, 1993. Glob. Plan. Change, 7, 145-156.