| Summary: | Young terrestrial planets can capture or outgas hydrogen-rich atmospheres with tens to hundreds of bars of H[subscript 2], which persist for 100 Myr or longer. Although the earliest habitable conditions on Earth and terrestrial exoplanets could thus arise while the atmosphere is still dominated by H[subscript 2], the climatic effects of H[subscript 2] remain poorly understood. Previous work showed that H[subscript 2] induces strong greenhouse warming at the outer edge of the habitable zone. Here we use a 1D radiative-convective model to show that H[subscript 2] also leads to strong warming near the inner edge of the habitable zone. Unlike H[subscript 2]'s greenhouse warming at the outer edge, however, its effect near the inner edge is driven by thermodynamics: H[subscript 2]'s large thermal scale height allows the atmosphere to store more water vapor than either a pure H[subscript 2]O atmosphere or an atmosphere with a heavy background gas, such as N[subscript 2] or CO[subscript 2], thereby amplifying the greenhouse effect of H[subscript 2]O. Using idealized gray calculations, we then present a general argument for how different background gases affect the inner edge of the habitable zone. H[subscript 2] stands out for its ability to induce novel "soufflé" climates, which further support its warming effect. Our results show that if the earliest conditions on a planet near the inner edge of the habitable zone were H[subscript 2]-rich, they were likely also hot: 1 bar of H[subscript 2] is sufficient to raise surface temperatures above 340 K, and 50 bar of H[subscript 2] are sufficient to raise surface temperatures above 450 K.
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