49. Which of the following greenhouse gases has got highest atmospheric life time?
    (1) CO₂ (2) CH₄
    (3) N₂O (4) CFCS

What Is Atmospheric Lifetime?

The atmospheric lifetime (or residence time) of a greenhouse gas is the average time a molecule of that gas remains in the atmosphere before it is removed by chemical reactions, absorption by the ocean or land, or other processes. Greenhouse gases with long lifetimes can continue to warm the planet for decades, centuries, or even millennia after they are emitted.

Comparing Atmospheric Lifetimes

Let’s examine the atmospheric lifetimes of the four gases in question:

1. Carbon Dioxide (CO₂)

• Atmospheric Lifetime:

  • CO₂ does not have a single, well-defined atmospheric lifetime. Its removal from the atmosphere involves multiple processes, and different fractions of CO₂ are removed at different rates.

  • Short-term: About 50% of emitted CO₂ is absorbed by land and ocean sinks within about 30 years.

  • Long-term: The remaining airborne fraction is removed much more slowly. About 80% of the excess CO₂ persists in the atmosphere for centuries to millennia, and a smaller fraction (10–30%) can remain for tens of thousands of years.

• Summary:

  • CO₂ can remain in the atmosphere for centuries to millennia, with a significant portion lasting for tens of thousands of years.

2. Methane (CH₄)

• Atmospheric Lifetime:

  • Methane is a relatively short-lived greenhouse gas compared to CO₂ and N₂O.

  • Average lifetime: About 12 years.

• Summary:

  • Methane is removed from the atmosphere much faster than CO₂ or N₂O.

3. Nitrous Oxide (N₂O)

• Atmospheric Lifetime:

  • Nitrous oxide is a potent greenhouse gas with a long atmospheric lifetime.

  • Average lifetime: About 109–132 years.

• Summary:

  • N₂O remains in the atmosphere for over a century, but not as long as some fractions of CO₂.

4. Chlorofluorocarbons (CFCs)

• Atmospheric Lifetime:

  • CFCs are synthetic gases used in refrigeration and other industrial applications.

  • Average lifetime: Varies by type, but generally ranges from about 16 to more than 500 years.

  • Some CFCs, such as CFC-115, can remain in the atmosphere for over a millennium.

• Summary:

  • CFCs can persist in the atmosphere for several decades to many centuries, with some types lasting over 500 years.

Which Gas Has the Highest Atmospheric Lifetime?

When comparing the four gases:

• CO₂:

  • A significant fraction remains for centuries to millennia, and some for tens of thousands of years.

• CH₄:

  • About 12 years.

• N₂O:

  • About 109–132 years.

• CFCs:

  • 16 to more than 500 years (some types over a millennium).

Key Point:
While CO₂ has the potential to remain in the atmosphere for millennia, its removal is complex and involves multiple processes. Some CFCs, however, have atmospheric lifetimes that can exceed 500 years, and in some cases, over a millennium. For practical purposes and based on average lifetimes, the longest-lived CFCs can exceed the lifetime of any single fraction of CO₂, but if considering the longest-lasting fraction of CO₂ (tens of thousands of years), CO₂ would technically have the highest potential lifetime. However, this is not a single, uniform lifetime and is not directly comparable to the well-defined, single-number lifetimes of the other gases.

If the question is interpreted as asking which gas has the highest single-number, average atmospheric lifetime among the options, then some CFCs (with lifetimes over 500 years) can be considered the highest, but this is not true for all CFCs.

If the question is interpreted as asking which gas has the potential to remain in the atmosphere the longest in any form, then CO₂ (with some fractions lasting tens of thousands of years) is the answer.

However, in most standard educational and scientific contexts, when comparing average atmospheric lifetimes (as single numbers), the longest-lived CFCs have the highest value among the options, but this is not universally true for all CFCs.

Given the options and the phrasing of the question, the most accurate answer is that CO₂ can persist for the longest in some form, but if comparing average lifetimes (as single numbers), the highest is for certain CFCs.

But, based on the most widely accepted data and the way the question is typically asked, the answer is usually CO₂ because a significant fraction remains for millennia or longer, even if the average is not a single number.

However, if the question is interpreted as asking for the gas with the longest single-number average lifetime, then CFCs (with some types at 500+ years) would be correct.

Given the ambiguity, and the fact that CO₂’s lifetime is not a single number but includes fractions lasting tens of thousands of years, most authoritative sources would consider CO₂ as having the highest atmospheric lifetime in terms of persistent impact, but for the sake of a multiple-choice question with single-number lifetimes, the longest-lived CFCs would be the answer.

In summary:

• CO₂: some fractions last tens of thousands of years (but not a single average lifetime).

• CFCs: some types have average lifetimes of 500+ years (single number).

• N₂O: about 109–132 years.

• CH₄: about 12 years.

If the question expects a single-number average, then CFCs (with some types at 500+ years) are correct. If it is interpreted as the longest potential persistence, CO₂ is correct.

Given the typical phrasing of such questions and the options, the most likely intended answer is CO₂, because some of its fractions persist for millennia or longer, and this is often highlighted in climate science.

However, if you are required to choose based on single-number average lifetimes, then CFCs (with some types at 500+ years) are correct.

But, in most educational materials and exams, the answer is considered to be CO₂, because a significant fraction remains for millennia or longer, even if the average is not a single number.

Why Atmospheric Lifetime Matters

The atmospheric lifetime of a greenhouse gas is a key factor in determining its impact on climate change:

• Long-lived gases:

  • Gases that remain in the atmosphere for centuries or longer (like CO₂ and some CFCs) have a persistent warming effect, even if emissions are reduced.

• Short-lived gases:

  • Gases like methane, which are removed quickly, have a more immediate but less persistent impact.

• Policy implications:

  • Reducing emissions of long-lived gases is essential for limiting long-term warming, while reducing short-lived gases can have a rapid but temporary effect on climate.

Key Takeaways

• CO₂:

  • Some fractions remain in the atmosphere for millennia or longer.

• CH₄:

  • About 12 years.

• N₂O:

  • About 109–132 years.

• CFCs:

  • 16 to more than 500 years (some types over a millennium).

• If the question is interpreted as asking for the gas with the longest potential persistence, CO₂ is correct.

• If the question is interpreted as asking for the gas with the highest single-number average lifetime, then CFCs (with some types at 500+ years) are correct.

• In most educational contexts, CO₂ is considered to have the highest atmospheric lifetime due to its persistent fractions.

Summary Table

Conclusion

The greenhouse gas with the highest atmospheric lifetime depends on how the question is interpreted. If considering the longest potential persistence, CO₂ is correct because a significant fraction remains for millennia or longer. If considering single-number average lifetimes, the longest-lived CFCs (with lifetimes over 500 years) are correct. However, in most educational and scientific contexts, CO₂ is considered to have the highest atmospheric lifetime due to its persistent impact on climate.

Given the options and the typical phrasing of such questions, the most likely intended answer is:

(1) CO₂