The central theme of climate change has, for many years, been global warming. Temperatures rise because the earth collects more thermal energy than it dissipates. Where does this heat which keeps our planet alive come from, and what is the earth’s heat budget? 

The sun lies 150 million kilometers from earth, and temperatures on the surface of the sun average 5,500^oC. So, radiant energy from the sun warms the surface of the planet, and without the sun, life on earth as we know it would not exist. However, there is another source of heat closer to us, beneath our feet. The earth acts as a heat engine with temperatures in its core that are hotter than the surface of the sun.

A well drilled into the subsurface records a rise in temperature with increasing depth below the earth’s surface. So, from our stationary perspective on the earth’s surface, we think of the temperature as increasing downward. However, there is another perspective available to us. High temperatures in the interior of the earth decrease away from the core as thermal energy passes to the surface. These outwardly decreasing temperatures reflect the fact that earth transfers its heat from the core to the surface, and then from the surface into space.

The earth’s core is hot, and current estimates place the temperature at the inner core boundary at about 6,000 ^oC. This temperature is slightly higher than temperatures on the surface of the sun. The primary sources of heat in the interior of the earth are remnant heat from when the planet formed, and heat from the decay of radioactive elements.

Earth’s interior heat budget

The earth formed some 4.6 billion years ago as stardust and cosmic debris collected into a gravity well. The energy released during this process created a molten planet, that has since cooled a bit. 

The temperature in deep space is 2.7 Kelvin (-270.45 ^oC). Stars and planets only occupy about 4% of the universe, so heat continually moves from the interiors of these hot celestial bodies towards the frigid cold of deep space. The earth cooled after its formation by dissipating its thermal energy into space. Currently, the earth cools at a rate of about 100 ^oC every billion years. As it cools, the solid inner core grows larger. 

So, the interior of the earth is hot from the remnant thermal energy of formation. However, the second source of heat is equally important. Estimates of the earth’s heat budget indicate that about 54% of the heat flowing from the earth’s interior today is from radiogenic decay. The decay of uranium-238, thorium-232, and potassium-40, continually produces about 24 terawatts of thermal energy (24 trillion Joules of thermal energy per second). The energy produced by radioactive decay each year is about 10 times greater that than the yearly energy consumption of the world. 

Earth’s heat budget and global warming

The total heat budget on the surface of the earth consists of incoming solar radiation and outgoing thermal energy from the earth’s interior. Of course, heat provided by the sun is thousands of times greater than the amount of heat coming from inside the planet. So the sun is the primary warming agent for the earth’s surface. 

Global warming reflects the dynamics of heat dissipation. If the earth had no atmosphere, the average surface temperature of the planet would be about minus 20 ^oC. The atmosphere is like a big duvet that traps heat. Greenhouse gases are the feathers inside the duvet.  The more feathers stuffed into the duvet, then the more heat it traps. 

Incoming solar radiation and outgoing heat from the earth’s interior are at about the same levels as they have been for thousands of years. What has changed is that human activities have injected more greenhouse gases into the atmosphere, thus increasing the capacity of the atmosphere to trap heat. Less heat can leave the planet, so the total heat content of earth increases, and then temperatures rise — global warming in a nutshell.

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Sources:

Earth’s Core 1,000 Degrees Hotter Than Expected: https://www.livescience.com/29054-earth-core-hotter.html Also:

Core: https://www.nationalgeographic.org/encyclopedia/core/ Also:

Why is the earth’s core so hot? And how do scientists measure its temperature?: https://www.scientificamerican.com/article/why-is-the-earths-core-so/ Also:

Earth Still Retains Much of Its Original Heat: https://www.sciencemag.org/news/2011/07/earth-still-retains-much-its-original-heat Also:

Ask Smithsonian: What’s the Deepest Hole Ever Dug?: https://www.smithsonianmag.com/smithsonian-institution/ask-smithsonian-whats-deepest-hole-ever-dug-180954349/ Also:

Science: An Elementary Teacher’s Guide/Energy, work, power, heat  https://en.wikibooks.org/wiki/Science:_An_Elementary_Teacher’s_Guide/Energy,_work,_power,_heat Also:

Heat, work and energy tutorial – essentials as specific heat  https://www.engineeringtoolbox.com/heat-work-energy-d_292.html Also:

Feature Image: Cut-away Diagram of Earth’s Interior (By Dixon Rohr – NASA) This file is in the public domain in the United States because it was solely created by NASA