Most Earth-system simulations run on conventional CPUs in 64-bit double precision floating-point numbers Float64, although the need for high-precision calculations in the presence of large uncertainties has been questioned. Fugaku, currently the world’s fastest supercomputer, is based on A64FX microprocessors, which also support the 16-bit low-precision format Float16. We investigate the Float16 performance on A64FX with ShallowWaters.jl, the first fluid circulation model that runs entirely with 16-bit arithmetic. The model implements techniques that address precision and dynamic range issues in 16 bit. The precision-critical time integration is augmented to include compensated summation to minimize rounding errors. Such a compensated time integration is as precise but faster than mixed-precision with 16 and 32-bit floats. As subnormals are inefficiently supported on A64FX the very limited range available in Float16 is 6.10-5 to 65504. We develop the analysis-number format Sherlogs.jl to log the arithmetic results during the simulation. The equations in ShallowWaters.jl are then systematically rescaled to fit into Float16, using 97% of the available representable numbers. Consequently, we benchmark speedups of 3.8x on A64FX with Float16. Adding a compensated time integration the speedup is 3.6x. Although ShallowWaters.jl is simplified compared to large Earth-system models, it shares essential algorithms and therefore shows that 16-bit calculations are indeed a competitive way to accelerate Earth-system simulations on available hardware.

Growth in aviation contributes more to global warming than is generally appreciated because of the mix of climate pollutants it generates: aviation contributed approximately 4% to observed human-induced global warming to date, despite being responsible for only 2.4% of global annual emissions of CO 2. Aviation is projected to have caused a total of about 0.1˚C of warming by 2050, half of it to date and the other half over the next three decades. Should aviation’s pre-COVID growth resume, the industry will contribute a 6-17% share to the remaining 0.3-0.8˚C to not exceed 1.5-2˚C of global warming. Under this scenario, the reduction due to COVID-19 to date is small and is projected to only delay aviation’s warming contribution by about 5 years. But the leveraging impact of growth also represents an opportunity: Aviation’s contribution to further warming would be immediately halted by either a sustained annual 2.5% decrease in flights under the existing fuel mix, or a transition to a 90% carbon-neutral fuel mix by 2050.

The need for high precision calculations with 64-bit or 32-bit floating-point arithmetic for weather and climate models is questioned. Lower precision numbers can accelerate simulations and are increasingly supported by modern computing hardware. This paper investigates the potential of 16-bit arithmetic when applied within a shallow water model that serves as a medium complexity weather or climate application. There are several 16-bit number formats that can potentially be used (IEEE half precision, BFloat16, posits, integer and fixed-point). It is evident that a simple change to 16-bit arithmetic will not be possible for complex weather and climate applications as it will degrade model results by intolerable rounding errors that cause a stalling of model dynamics or model instabilities. However, if the posit number format is used as an alternative to the standard floating-point numbers the model degradation can be reduced to a tolerable minimum. Furthermore, a number of mitigation methods, such as rescaling, reordering and mixed-precision, are available to make model simulations resilient against a precision reduction. If mitigation methods are applied, 16-bit floating-point arithmetic can be used successfully within the shallow water model. The results show the potential of 16-bit formats for at least parts of complex weather and climate models where rounding errors would be entirely masked by initial condition, model or discretization error.