  Climate models must simulate hundreds of future scenarios for hundreds of
years at coarse resolutions, and a handful of high-resolution decadal
simulations to resolve localized extreme events. Using Oceananigans.jl, written
from scratch in Julia, we report several achievements: First, a global ocean
simulation with breakthrough horizontal resolution -- 488m -- reaching 15
simulated days per day (0.04 simulated years per day; SYPD). Second,
Oceananigans simulates the global ocean at 488m with breakthrough memory
efficiency on just 768 Nvidia A100 GPUs, a fraction of the resources available
on current and upcoming exascale supercomputers. Third, and arguably most
significant for climate modeling, Oceananigans achieves breakthrough energy
efficiency reaching 0.95 SYPD at 1.7 km on 576 A100s and 9.9 SYPD at 10 km on
68 A100s -- the latter representing the highest horizontal resolutions employed
by current IPCC-class ocean models. Routine climate simulations with 10 km
ocean components are within reach.
