I need to test the hypothesis that the annual global carbon sink is weakening over time by testing whether this hypothesis is supported by the data through comparing decadal means using a two-tailed t-test.

I have data of carbon sink uptake 1961-2011, so split the decades into 1961-1970, 1971-1980 etc and took the mean of the sink values and now have 2 columns like this:

1961-1970 1.410792418
1971-1980 1.559115845
1981-1990 1.800177877
1991-2000 2.212384716
2001-2010 2.500734866

With what variables do I perform the t-test? And how do I know my confidence level?

Thanks so much in advance!