Unsettled Science: Are We Really Measuring Earth's Energy Imbalance Accurately?

If We Can’t Measure the Imbalance, Should We Be Making Radical Policy Shifts?

The greenhouse effect is often cited as the primary driver behind global warming, with anthropogenic greenhouse gases (GHGs), like carbon dioxide (CO2), playing a central role in creating an imbalance in Earth's energy system. The mechanism behind the greenhouse effect is relatively straightforward: solar radiation enters the Earth’s atmosphere, where some of it is absorbed by the surface, and the rest is re-radiated as infrared energy. GHGs absorb and re-emit part of this energy, trapping heat in the atmosphere and warming the planet.

While the concept of the greenhouse effect is well understood, its exact contribution to global warming, particularly due to human activities, is much more uncertain. Two influential papers from 2002 by Wielicki et al. (no relation to me) and Chen et al. bring this uncertainty to the forefront by showing significant gaps in our ability to measure and quantify the Earth's energy balance. This raises a critical question: should we make drastic, sweeping policy decisions based on an effect that we are struggling to accurately measure?

The Greenhouse Effect: A Brief Overview

The greenhouse effect is the process by which greenhouse gases like carbon dioxide (CO2), methane (CH4), and water vapor trap heat in the Earth’s atmosphere, preventing it from escaping into space. Solar radiation enters the Earth’s atmosphere, where a portion is absorbed by the Earth’s surface and the rest is radiated back as infrared energy. Greenhouse gases absorb and re-emit some of this infrared energy, effectively warming the atmosphere and, consequently, the Earth's surface. Without the greenhouse effect, Earth's surface temperature would be significantly colder, around -18°C (0°F), compared to the average of approximately 15°C (59°F) that we experience today.

This figure shows the absorption bands in the Earth's atmosphere (middle panel) and the effect that this has on both solar radiation and upgoing thermal radiation (top panel). Individual absorption spectrum for major greenhouse gases plus Rayleigh scattering are shown in the lower panel. Source: https://en.m.wikipedia.org/wiki/File:Atmospheric_Transmission.svg

While the greenhouse effect is a natural and necessary phenomenon, the primary concern revolves around the "enhanced greenhouse effect" — the hypothesis that human activity, particularly the burning of fossil fuels, has elevated concentrations of GHGs, thus leading to a warming planet (global warming). Quantifying this anthropogenic effect is central to current climate science and policy decisions, but the two papers in question highlight critical gaps in our ability to measure and understand this effect.

Earths Energy Balance

Earth's energy balance refers to the equilibrium between incoming solar radiation and the outgoing thermal energy radiated back into space. Any imbalance between these two can lead to warming or cooling of the Earth. According to many climate models, the Earth is currently experiencing a positive energy imbalance—meaning more energy is being absorbed than emitted—largely attributed to human activities like the burning of fossil fuels, which increases the concentration of GHGs in the atmosphere. This imbalance is currently estimated to be around 0.6–0.8 W/m². However, such estimates are far from certain, and the complexity of Earth's energy budget introduces many variables that are difficult to quantify.

The papers by Wielicki et al. and Chen et al. reveal the uncertainties in measuring Earth's energy balance, particularly in relation to cloud feedbacks and tropical energy budgets. Both papers show that natural variability in energy distribution, along with difficulties in measuring the Earth's radiative output, complicates our understanding of how much GHGs are contributing to global warming.

Earth’s energy budget describes the balance between the radiant energy that reaches Earth from the sun and the energy that flows from Earth back out to space. Credits: NASA

Wielicki et al. (2002): A Study of Tropical Variability

In their 2002 paper, Wielicki et al. present data from the Earth Radiation Budget Experiment (ERBE) and the Clouds and the Earth’s Radiant Energy System (CERES) satellites to analyze the radiative energy budget over the tropics.