It is widely accepted that long-term climate variability is controlled primarily by CO 2 with other greenhouse gases and planetary albedo playing an important, but subsidiary, role. On million-year timescales, CO 2 is sourced predominantly from volcanic outgassing, consumed predominantly by silicate weathering, and modulated by the silicate weathering feedback (e.g., Berner, 2006). As summarized by Saltzman (2017), the relative importance of CO 2 sources and sinks as the driver of transitions between non-glacial and glacial climate regimes is a matter of debate and proxies for both come with caveats. As silicate weathering is dependent on latitude, topography, temperature, and precipitation, improved geological and paleogeographic constraints offer the potential to better predict how silicate weathering has changed through time. In Swanson-Hysell and Macdonald (2017), we examined these constraints and argued that increased planetary weatherability led to lower steadystate CO 2 through the exhumation of arc-related lithologies during the Taconic orogeny. We emphasized the importance of exhumation occurring in an east-west orientation within the equatorial humid belt, rather than the arid subtropics, as well as the high weatherability of exhumed arc-ophiolite rocks. Both factors enhance the plausibility of the Kump et al. (1999) hypothesis that the Taconic orogeny significantly modulated Ordovician climate. We also argued that compiled proxies for local weathering provenance ( Nd ) are consistent with this volcanicweathering hypothesis, as are global records that are influenced by weathering inputs (e.g., 87 Sr/ 86 Sr;Berner, 2006). We acknowledged that the Taconic orogeny was not the sole change in tectonic boundary conditions at the time, and other arc systems, such as those in the Central Asian Orogenic Belt (CAOB), likely enhanced global weatherability. However, most of the ophiolite-bearing sutures in the CAOB are Late Paleozoic in age, not Cambrian (Furnes et al., 2014). Ordovician large slab ophiolites in the Caledonian-Appalachian belt, which extend through Norway to the M'Clintock orogen on Ellesmere Island, are among the largest and best preserved in the geological record despite erosion and structural reworking in subsequent orogenesis (Furnes et al., 2014).Landing (2018) argues that fluctuations in Ordovician climate were largely not controlled by CO 2 , but instead by non-glacial eustatic sea level. His hypothesis is that the Ordovician was characterized by periods of "hyperwarming" when high eustatic sea level lowered planetary albedo and increased water vapor in the atmosphere. He envisions that these non-glacial global eustatic sea-level changes were large, rapid, and repeated (Landing, 2012, his figure 2) and that non-glacial global eustatic sea-level fall (with an unclear mechanism) initiated cooling that led to the Hirnantian glaciation (Landing, 2018). This hypothesis has a number of weaknesses, with the most glaring being that it invokes large and rapid changes in ice-free global eustasy. Chan...