The global temperature trend observed over the last century is largely the result of two opposing effects-cooling from aerosol particles and greenhouse gas warming. While the effect of increasing greenhouse gas concentrations on Earth's radiation budget is well constrained, that due to anthropogenic aerosols is not, partly due to a lack of observations. However, long-term surface measurements of changes in downward solar radiation (SDSR), an often used proxy for aerosol radiative impact, are available worldwide over the last half century. We compare SDSR changes from ∼1,400 stations to those from the Coupled Model Intercomparison Project Version 5 global climate simulations over the period 1961-2005. The observed SDSR shows a strong early downward trend followed by a weaker trend reversal, broadly consistent with historical aerosol emissions. However, despite considerable changes to known aerosol emissions over time, the models show negligible SDSR trends, revealing a lethargic response to aerosol emissions and casting doubt on the accuracy of their future climate projections.Plain Language Summary Observations of incoming solar radiation, as measured at approximately 1,400 surface stations worldwide, show a strong downward trend from the 1960s to the 1980s, followed by a weaker trend reversal thereafter. These trends are thought to be due to changes in the amount of aerosol particles in the atmosphere, and we find support for that here in the temporal evolution of anthropogenic aerosol emissions. This is expected because aerosol particles reflect and/or absorb sunlight back to space and have a net cooling effect on Earth's climate. However, we find that the current generation of climate models simulates negligible solar radiation trends over the last half century, suggesting that they have underestimated the cooling effect that aerosol particles have had on climate in recent decades. Despite this, climate models tend to reproduce surface air temperature over the time period in question reasonably well. This, in turn, suggests that the models are not sensitive enough to increasing greenhouse gas concentrations in the atmosphere, with important implications for their ability to simulate future climate.

Surface melting occurs during summer on the Antarctic and Greenland ice sheets, but the volume of stored surface meltwater has been difficult to quantify due to a lack of accurate depth estimates. NASA's ICESat-2 laser altimeter brings a new capability: photons penetrate water and are reflected from both the water and the underlying ice; the difference provides a depth estimate. ICESat-2 sampled Amery Ice Shelf on January 2, 2019 and showed double returns from surface depressions, indicating meltwater. For four melt features, we compared depth estimates from eight algorithms: six based on ICESat-2 and two from coincident Landsat-8 and Sentinel-2 imagery. All algorithms successfully identified surface water at the same locations. Algorithms based on ICESat-2 produced the most accurate depths; the image-based algorithms underestimated depths (by 30%-70%). This implies that ICESat-2 depths can be used to tune image-based algorithms, moving us closer to quantifying stored meltwater volumes across Antarctica and Greenland.

Antarctica's ice shelves regulate the flow of grounded ice to the ocean, through a process known as buttressing (Thomas, 1979). Under stable climate conditions, ice shelves remain approximately in equilibrium, gaining mass from ice flow across the grounding line and local snowfall, and losing mass through ocean melting at their bases (year-round), iceberg calving from their ice fronts (episodic) and surface melting (principally during summer). In recent decades, some Antarctic Peninsula ice shelves have experienced greater surface melting in response to increasing atmospheric temperatures (Barrand et al., 2013;Trusel et al., 2015). This has led to more extensive melt ponds, providing sufficient water volumes to drive so-called "hydrofracturing" (Weertman, 1973), sometimes leading to ice shelf collapse via an extreme disintegrative type of calving involving multiple hydrofractures (Banwell et al., 2013;Scambos et al., 2003;van den Broeke, 2005). In these regions, flow rates of the grounded ice have increased (Rignot et al., 2004;Scambos et al., 2003), due to loss of buttressing.Antarctic surface melting has been projected to double by 2050 (Gilbert & Kittel, 2021;Trusel et al., 2015), raising concerns about the stability of other ice shelves. This has renewed interest in monitoring surface