GARP SCR: Understanding Radiative forcing

 This question was posed by one of the group member Valentina Ricciuti from the Linkedin group (https://www.linkedin.com/groups/12842133/). 

What is Radiative forcing?

Defining Radiative forcing:

Radiative forcing is a fundamental concept in climate science used to measure the disturbance in Earth's energy balance caused by factors influencing our planet's climate. It's expressed in watts per square meter (W/m²) and represents the disparity between the solar energy absorbed by Earth and the thermal (infrared) energy radiated back into space. Positive radiative forcing indicates more energy being trapped, leading to global warming, often due to increased greenhouse gas concentrations. Negative radiative forcing signifies excess energy escaping into space, resulting in cooling, such as from volcanic eruptions. Radiative forcing helps quantify the impact of various factors on climate change, aiding in understanding their contributions to global temperature shifts and guiding predictions about future climate patterns.

Further explanation

Consider the below equation:

Hs = (He) + (Ha) + (Hr)

Where

1. Energy/Heat from the sum = (Hs)

2. Heat absorbed by earth's surface = (He)

3. Heat absorbed by the atmosphere = (Ha)

4. Heat reflected back into the space = (Hr)

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Now, consider the attached image and travel with me to about 100,000 to 110,000 years ago. I have shown this period between red lines in the image.

The period indicates ice age where the temperature is naturally decreasing, water is freezing and ice cover is increasing around the earth.

As there is more and more ice, what happens?

- Albedo effect increases and reflects more and more heat from the sun

- Overall biological activity is decreased, leading to overall reduction in CO2 (as most mechanisms emit Carbon dioxide).More details mentioned below

Let us look at our equation again:

Hs = (He) + (Ha) + (Hr)

Assume, (Hs) remains the same since the time earth was created.

Due to increased ice cover, (He) decreases

Due to less carbon dioxide, atmosphere absorbs less heat, (Ha) decreases

So, (Hr) increases , which means heat reflected is more.

As the result of all this temperature begins to drop or decrease.

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Let us move forward to just 25000 years ago, the ice cover vanished due to natural reasons. So, what is the impact?

- Due to decreased ice cover, albedo effect decreases ---> (He) increases

- As ice cover vanishes, there is an increase in the biological activities leading to higher carbon dioxide.

Due to more carbon dioxide, the atmosphere absorbs more heat, (Ha) decreases increases (typo as notified by Natascha through her comment on 1st Oct'23)

- So, (Hr) decreases , which means less heat reflected is more.

As the result of all this temperature begins to rise.

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Let us now travel to the 1900s where the industrial age has started, large scale industrialization, fossil fuels have increased. There are more greenhouse gases (GHG) in the atmosphere like CH4, NO, HFCs etc... in addition to increased amounts of Carbon dioxide.

Let us look at our equation again:

Hs = (He) + (Ha) + (Hr)

Ice cover is anyways less, this means (He) is more.

Due to GHG, the atmosphere is absorbing more heat , (Ha) is high.

Due to all this, (Hr) is further less. This means less and less heat is reflected.

This means More heat is absorbed and less and less reflected. This is leading to heating up.

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Hopefully, this should explain how the radiative forcing is changing over a period of time. This concept will also be useful in understanding "Representative concentration pathways" (RCPs)

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Coming to the question as to why carbon dioxide reduced during the ice age. Below is an explanation with few factors that led to decrease in CO2.

During ice ages, carbon dioxide (CO2) levels in the Earth's atmosphere decrease for several interconnected reasons. This decrease in atmospheric CO2 is a critical factor in amplifying the cooling associated with ice ages. Here's an overview of why this decrease occurs:

1.Ocean Carbon Storage:

- Cold water can hold more dissolved gases, including CO2, than warm water. During ice ages, the Earth's climate cools, causing the ocean's surface waters to become colder.

- As the surface waters cool, they absorb more CO2 from the atmosphere, leading to increased dissolution of CO2 in the oceans. 

2.Expansion of Polar Ice Sheets:

- Ice sheets and glaciers expand during ice ages, covering large land areas. As ice accumulates on land, it effectively locks up water that would otherwise be in the liquid form in the oceans.

- This leads to a drop in sea level, which exposes continental shelves (landmasses that are usually submerged under water). The exposed shelves become carbon-rich terrestrial environments that are less conducive to storing CO2 compared to the deep ocean.

3.Reduced Biological Activity:

- Ice ages are associated with colder climates and altered weather patterns. These conditions can lead to reduced primary productivity in the oceans.

- Lower biological productivity means that less organic matter, which contains carbon, is being produced and ultimately sinking to the ocean floor. 

- Also, there are organisms who can't survive the change in weather and may extinct. This also impacts the amount of CO2.

4. Positive Feedback Mechanisms:

- The initial drop in CO2 levels can trigger positive feedback mechanisms that reinforce cooling. For example, lower CO2 levels contribute to cooler temperatures through the greenhouse effect, further promoting ice sheet growth and cooling.

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Please share your feedback/comments/different point of views/corrections in the comment section of this post.

Other useful information from the same chapter

Climate measurement proxies

Global warming

El nino

What is Albedo Effect?

Few questions to try from Chapter 1

Question of the day based on Chapter-1 

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