Social scientist Luke Kemp indicated that alternative sources of energy, such as solar panels, have low EROI because they have low energy density, meaning they require a lot of land, and require substantial amounts of rare earth metals to produce. Charles Hall and his colleagues reached the same conclusion. While there is no on-site pollution, the EROI of renewable energy sources may be too low for them to be considered a viable alternative to fossil fuels, which continue to provide the majority of the energy consumed by humanity (60–65% as of 2014). Moreover, renewable energy is intermittent and requires large and expensive storage facilities in order to be a base-load source for the power grid (20% or more). In that case, its EROI would be even lower. Paradoxically, therefore, expansions of renewable energy require more consumption of fossil fuels. For Hall and his colleagues, whereas human societies in the previous few centuries could solve or at least alleviate many of their problems by making technological innovations and by consuming more energy, contemporary society faces the difficult challenge of declining EROI for its most useful energy source, fossil fuels, and low EROI for alternatives.