Substantial extension of the lifetime of the terrestrial biosphere

Approximately one billion years (Gyr) in the future, as the Sun brightens, Earth's carbonate-silicate cycle is expected to drive CO$_2$ below the minimum level required by vascular land plants, eliminating most macroscopic land life. Here, we couple global-mean models of temperature- and CO$_2$-dependent plant productivity for C$_3$ and C$_4$ plants, silicate weathering, and climate to re-examine the time remaining for terrestrial plants. If weathering is weakly temperature-dependent (as recent data suggest) and/or strongly CO$_2$-dependent, we find that the interplay between climate, productivity, and weathering causes the future luminosity-driven CO$_2$ decrease to slow and temporarily reverse, averting plant CO$_2$ starvation. This dramatically lengthens plant survival from 1 Gyr up to $\sim$1.6-1.86 Gyr, until extreme temperatures halt photosynthesis, suggesting a revised kill mechanism for land plants and potential doubling of the future lifespan of Earth's land macrobiota. An increased future lifespan for the complex biosphere may imply that Earth life had to achieve a smaller number of ``hard steps'' (unlikely evolutionary transitions) to produce intelligent life than previously estimated. These results also suggest that complex photosynthetic land life on Earth and exoplanets may be able to persist until the onset of the moist greenhouse transition.