To examine the influence of mountain uplift on the long-term carbon cycle, we used geochemical, hydrologic, and suspended-load data for 12 streams draining the New Zealand Southern Alps to quantify rates of erosion, weathering, and atmospheric CO2 consumption. Rapid uplift in the western Southern Alps elevates mechanical erosion rates by a factor of ∼13 relative to those on the tectonically stable eastern side [125 × 108 vs. 9.4 × 108 g/(km2·yr), respectively]. Similarly, the average chemical weathering rate is ∼5 times higher on the western compared to eastern side of the mountain range [9.8 × 107 vs. 2.0 × 107 g/(km2·yr), respectively]. However, because the proportion of stream-water Ca2+ and Mg2+ from carbonate weathering increases as the rate of mechanical erosion increases, the long-term atmospheric CO2 consumption rate on the western side is ∼2 times higher than that on the eastern side [14 × 104 vs. 6.9 × 104 mol/(km2·yr), respectively] and only ∼1.5 times higher than the global mean value [∼9 × 104 mol/(km2·yr)]. Data for major world rivers (including Himalayan rivers) provide a consistent interpretation regarding the relationship between mechanical erosion intensity and the ratio of silicate to carbonate weathering. Thus, we conclude that mountain building increases atmospheric CO2 consumption rates by only a factor of ∼2, which is much lower than previous estimates.