AN ANALOG EARTH CLIMATE MODEL

Earth’s climate is governed by the radiative power of the sun as well as the heat retention and convective cooling of the atmosphere. I have constructed an analog earth model for an upper level undergraduate climate class that simulates mean climate. The ‘earth’ is a hollow, black, Aluminum sphere (4 cm diameter), illuminated by two opposite optic fibers, with light focused on the sphere by a set of lenses. The sphere is encased in a double-walled aluminum cylinder (34 cm diameter by 26 cm high) with water cooling jackets at the top, bottom, and sides. The cylinder can be filled with a gas of choice or in low vacuum. The temperature of the sphere, atmosphere and walls is monitored with thermocouples. Undergraduate E&ES students at Wesleyan University have run experiments with dry air, pure CO2, N2 and Ar at 1 atmosphere. To begin an experiment, the lights are flipped on, the temperature acquisition routine is activated (reading every 10 seconds), and the sphere starts to warm up until an equilibrium temperature has been reached. The lights are then flipped off and the cooling sequence towards ambient is registered. For each time increment the radiative heat loss of the sphere is calculated from the Stefan Boltzman expression using the observed temperature at that time. The heating of the ‘earth sphere’ is accounted for in the energy balance equation by applying the temperature increase per time increment with the specific heat of Aluminum. The remaining energy term is the sum of the convective cooling and greenhouse effect. The heat budgets of the cooling trajectories were calculated analogous, with radiative and convective cooling causing the temperature drop per time increment. Equilibrium temperatures of 50-70 C were reached, with ambient temperature at 22 C. Somewhat surprising, experiments with radiatively neutral pure Argon gas yielded the highest equilibrium temperatures. Argon had the lowest specific heat of the gases used, and the observed equilibrium temperatures for different cell gases broadly scaled inversely with the heat capacity of those gases. The free convective cooling impacts the temperatures more than the heat trapping capacity of the gases for this scale model at 1 bar pressure. We plan more systematic experiments at higher gas filling pressures to estimate the greenhouse effects on equilibrium temperatures.