Climate Processes

What are the three most abundant gases in dry air, in order, with rough percentages?

Nitrogen N2 (~78%), oxygen O2 (~21%), argon Ar (~0.93%). CO2 comes next at only ~0.04%.

Why is argon NOT counted as a trace gas, while CO2 is?

Argon is ~0.93% of air, so it is a major constituent. Trace gases are those below ~0.1% (CO2, CH4, N2O, O3, Ne, He, H2, CFCs), and many of them trap heat or harm health.

What is the fifth most abundant gas in dry air (after N2, O2, Ar and CO2)?

Neon (~18 ppm).

Roughly what wavelengths are ultraviolet (UV), visible light and infrared (IR)?

UV is shorter than ~0.4 µm (400 nm), visible is ~0.4-0.7 µm, and infrared is longer than ~0.7 µm. 1 µm (micrometer) = 1,000 nm (nanometers).

Which parts of the infrared are near-IR (NIR), shortwave IR (SWIR) and thermal IR?

Near-IR (~0.7-1.3 µm) and SWIR (~1.3-3 µm) are mostly reflected sunlight. Thermal IR (the 3-5 µm mid-IR and ~8-15 µm) is heat that Earth itself emits.

Why is sunlight called "shortwave" radiation and Earth's emitted radiation "longwave"?

The hot Sun (~5,800 K) emits mostly at ~0.2-4 µm (peak near 0.5 µm, visible). The cool Earth (~255-288 K) emits at ~4-100 µm (peak near 10 µm, thermal infrared).

What does a greenhouse gas do to incoming sunlight and to Earth's outgoing infrared?

It lets most sunlight (shortwave) pass through but absorbs outgoing infrared (longwave) and re-emits part of it back down, which warms the surface.

What is the "atmospheric window"?

The ~8-14 µm range of thermal infrared where water vapor and CO2 absorb very little, so heat from the surface escapes straight to space (ozone's 9.6 µm band sits inside it).

Why are N2 and O2, which make up 99% of air, not greenhouse gases?

A molecule absorbs infrared only if its vibrations shift its electric charge around. Two identical bonded atoms cannot do that, so N2 and O2 barely absorb. H2O, CO2, CH4, N2O, O3 and halogenated gases can.

What does "ppm" mean for a gas like CO2?

Parts per million by mole: molecules of that gas per million molecules of dry air.

What does an N2O concentration of 334 ppb physically mean?

334 of every billion air molecules are N2O (ppb = parts per billion, and 1 ppm = 1,000 ppb).

Convert 450 ppm to percent.

0.045%. Divide ppm by 10,000, since 1% = 10,000 ppm.

In which atmospheric layer are almost all greenhouse gases and all weather found?

The troposphere (surface to ~8 km at the poles, ~16-18 km at the equator), which holds ~75-80% of the air's mass and nearly all water vapor.

Which atmospheric layer sits directly above the troposphere, and how does its temperature change with height?

The stratosphere (from ~8-16 km up to ~50 km). Its temperature rises with height, and it holds the ozone layer.

At what altitude range is stratospheric ozone concentrated?

About 15-35 km (peak density near 20-25 km).

Why does temperature increase with height in the stratosphere?

Ozone absorbs solar UV and turns it into heat, so the upper stratosphere is heated most.

What makes a gas "well-mixed" in the atmosphere?

Its lifetime is much longer than the ~1 year it takes air to mix around the globe, so its concentration is nearly the same everywhere.

Which greenhouse gases are well-mixed?

CO2, CH4, N2O, CFCs, HFCs and other halogenated gases, and SF6 (all last years or longer).

True or false: water vapor is a well-mixed greenhouse gas.

False. It lasts only ~9 days and ranges from near 0% to ~4%, so it varies strongly in space and time.

Which atmospheric species are highly variable rather than well-mixed?

Water vapor, aerosols, NO2, SO2, ozone and CO (CO lives ~1-2 months). Short lifetimes keep them clustered near their sources.

How long does air take to mix between the Northern and Southern Hemispheres, and why so slow?

About 1 year, because the ITCZ (the rainy belt near the equator where the northeast and southeast trade winds, the steady tropical easterlies, meet) acts as a barrier. Mixing around one latitude band takes only weeks.

What does radiative forcing measure, and which sign means warming?

The change in Earth's energy balance caused by some factor, in W/m² (watts per square meter). Positive = more energy kept = warming. Negative = cooling.

What is the difference between a positive and a negative feedback?

A positive feedback amplifies the initial change. A negative feedback dampens it and pushes the system back toward balance.

What timescale separates climate from weather?

Weather covers hours to days. Climate is the long-term average, conventionally over 30 years (years to decades).

What does a gas's global warming potential (GWP) compare?

The warming from 1 kg of the gas versus 1 kg of CO2 over a set time horizon, usually 100 years. CO2 = 1 by definition.

Which two properties of a greenhouse gas determine its GWP?

Its radiative efficiency (how strongly each added unit absorbs infrared) and its atmospheric lifetime (how long it stays).

CO2 is increasing everywhere at once, but surface warming is uneven by region. Why?

CO2 is well-mixed, but circulation, ocean heat uptake and local feedbacks differ by region (land and the Arctic warm fastest).

What is albedo, and which surfaces have high and low albedo?

The fraction of sunlight a surface reflects (0 to 1). Fresh snow (~0.8-0.9) and sea ice (~0.5-0.7) are high, open ocean (~0.06) and forest (~0.1-0.15) are low. Earth overall is ~0.30.

About how much faster has the Arctic warmed than the global average, and what is the main reason?

About 3-4 times faster since 1979 (Arctic amplification), largely from the ice-albedo feedback: melting bright ice exposes dark ocean and land that absorb more sunlight, which melts more ice.

What is the NDVI formula, and why is it high for healthy vegetation?

NDVI = (NIR - Red) ÷ (NIR + Red), with NIR = near-infrared. Chlorophyll absorbs red light and the leaf's inner cell structure strongly reflects NIR, so healthy leaves give a big positive difference.

What is the most abundant greenhouse gas, and what is second?

Water vapor (~0.4% on average, up to ~4% in the humid tropics), then CO2 (~0.04%).

Roughly what shares of today's greenhouse effect come from water vapor, clouds and CO2?

Water vapor ~50%, clouds ~25%, CO2 ~20%, all other gases ~5%.

Why is water vapor called a feedback rather than a forcing?

Its amount is set by temperature: extra vapor rains out within ~9 days, and warmer air holds more. So it amplifies warming started by CO2 but cannot drive it alone.

Which greenhouse gas has the greatest concentration variability and the shortest residence time?

Water vapor (it stays only ~9-10 days before raining out).

What does the Clausius-Clapeyron relation say about water vapor and temperature?

The air's water vapor capacity (saturation vapor pressure) rises exponentially with temperature, about 7% per 1 °C.

As the planet warms, what happens to total atmospheric water vapor, and why does that matter?

It increases about 7% per °C (relative humidity stays roughly constant), which strengthens the greenhouse effect: the water vapor feedback.

Which highly variable gas dominates humidity and has absorption bands all across the infrared?

Water vapor (bands near 0.94, 1.4, 1.9, 2.7 and 6.3 µm, plus a broad band beyond ~20 µm).

What is atmospheric CO2 today, and what was it before industrialization?

About 425 ppm (2025, rising ~2.5-3 ppm per year) vs ~280 ppm pre-industrial, about 1.5x higher.

Using a Keeling curve (about 317 ppm in 1960 and 414 ppm in 2020), estimate the average rate of CO2 increase.

(414 - 317) ÷ 60 yr ≈ 1.6 ppm per year. Read the smoothed annual-mean line, not seasonal peaks.

What kind of growth does atmospheric CO2 show: linear, exponential, logarithmic or inverse?

Exponential (accelerating). The yearly rise grew from ~0.8 ppm/yr in the 1960s to ~2.5 ppm/yr recently, so the curve bends upward.

Which greenhouse gas has caused the largest increase in radiative forcing since 1750?

CO2 (~2.2 W/m², about two-thirds of the well-mixed greenhouse gas forcing). Methane is second (~0.5 W/m²).

Why does CO2 have no single atmospheric lifetime?

Different sinks remove extra CO2 at different speeds (surface ocean decades, deep ocean centuries, rock weathering millennia), and ~20% of a pulse lingers for thousands of years.

Why is CO2 a few ppm higher in the Northern Hemisphere than in the Southern?

About 90% of fossil fuel CO2 is emitted in the NH, and air takes ~1 year to cross the equator, so the gradient persists. The ocean-dominated SH also absorbs CO2.

How do scientists know the CO2 rise comes from fossil fuels (the isotope fingerprint)?

Fossil carbon has no carbon-14 and little carbon-13 (heavier forms, or isotopes, of carbon), and atmospheric CO2's 14C and 13C/12C have both been falling (the Suess effect). O2 is also dropping in step with burning.

How do you match the CO2, CH4 and N2O curves on a concentration-history graph?

By units and size of rise: CO2 in ppm (280 → ~425), CH4 in ppb with the steepest relative rise (~720 → ~1,930), N2O in ppb with the smallest, steadiest rise (~270 → ~337).

Who started the Keeling curve, where, and when?

Charles David Keeling, at Mauna Loa Observatory, Hawaii, in 1958 (about 315 ppm then).

Why was Mauna Loa chosen for measuring background CO2?

It sits ~3,400 m up on a remote mid-Pacific volcano, far from cities, forests and continents, so it samples well-mixed background air.

The Keeling curve combines two signals. What are they?

A long-term rise (fossil fuels, land use) plus a yearly sawtooth from the seasonal cycle of Northern Hemisphere plants.

In which month does Mauna Loa CO2 peak each year?

About May, after a whole NH winter of respiration and decay and just before spring leaf-out starts pulling CO2 down.

In which months is Northern Hemisphere CO2 lowest, and why?

About September-October, after a whole summer of photosynthesis drawing CO2 down.

Keeling seasonal cycle, step 1: what happens to CO2 during NH spring and summer?

Leaves grow and photosynthesis outpaces respiration, pulling CO2 down.

Keeling seasonal cycle, step 2 (after the summer drawdown): what happens during NH fall and winter?

Photosynthesis slows while respiration and decay of leaves and soil continue, so CO2 climbs until about May.

Why does the NH plant cycle control the global CO2 seasonal cycle, even though the SH has opposite seasons?

About two-thirds of land (and most land plants) is in the NH, so its signal swamps the small, opposite SH cycle.

Where is the CO2 seasonal swing largest, and where is it smallest?

Largest at high northern latitudes (~15-20 ppm in Alaska), ~6-7 ppm at Mauna Loa, and only ~1 ppm at the South Pole.

What is the "breathing" of Earth that OCO-2 (NASA's CO2-mapping satellite) helps observe?

The seasonal rise and fall of CO2 from NH plants: summer photosynthesis draws it down, winter respiration releases it.

True or false: atmospheric CO2 peaks in Northern Hemisphere summer.

False. NH summer is when plants pull CO2 down. It peaks around May and bottoms out around September-October (one test key got this wrong).

On an OCO-2 map, where is XCO2 (column-average CO2) lowest in late summer?

Over Northern Hemisphere land, especially the boreal (cold northern conifer) and temperate forests, after the growing season.

What effect do forest fires have on local atmospheric CO2 (for example OCO-2 XCO2)?

It increases, most at and just downwind of the fire core where smoke is densest.

What is atmospheric methane today versus pre-industrial?

About 1,930 ppb (~1.9 ppm) vs ~720 ppb, about 2.6x higher: the largest relative rise of the major greenhouse gases.

How long does methane stay in the atmosphere?

About 10-12 years.

What is the primary sink of atmospheric methane?

Reaction with hydroxyl radicals (OH, a very reactive molecule made by sunlight, the air's "detergent") in the troposphere: CH4 + OH → CH3 + H2O (~90% of removal).

What is the largest natural source of methane?

Wetlands (~30% of all methane emissions), where microbes break down organic matter without oxygen.

Which human activity contributes the most methane?

Agriculture: livestock digestion and manure plus rice paddies (~40% of human CH4). Fossil fuels are ~35%, landfills and waste ~20%. Humans cause ~60% of all CH4 emissions.

Give natural and human sources of methane.

Natural: wetlands, termites, geologic seeps, permafrost thaw. Human: livestock, rice paddies, landfills, oil and gas leaks, coal mining, biomass burning.

Which greenhouse gas comes from livestock digestion, rice paddies and wetlands and has a GWP much higher than CO2?

Methane (CH4).

Plants and animals release carbon mainly as what gas? Which carbon gas is made almost only by microbial decomposers?

CO2, by respiration. Methane, made by methanogens (archaea, often called bacteria) in oxygen-free places such as wetlands, rice paddies, landfills and cow stomachs.

Why does an added molecule of CO2 trap less extra heat than an added molecule of a rare gas like methane?

CO2's main 15 µm band already absorbs nearly all the radiation at its center (it is "saturated"), so extra CO2 only widens the band edges. A rare gas's bands are far from saturated, so each new molecule counts fully.

Why is methane far more potent per molecule than CO2?

It absorbs near 7.7 µm, where little else absorbs, and it is ~200x less abundant than CO2, so its band is far from saturated and each added molecule traps much more heat.

What is methane's GWP over 100 years and over 20 years?

About 28-30 over 100 years and about 80 over 20 years.

Why is methane's 20-year GWP so much larger than its 100-year GWP?

It lasts only ~12 years, so its warming is packed into the early years, while long-lived CO2 keeps warming across the whole century.

If large methane leaks were plugged, how fast would the radiative forcing drop, and why?

Within about a decade, because methane's ~12-year lifetime lets its concentration fall quickly. Cutting CH4 is the fastest lever on near-term warming.

A methane time series shows rapid jumps followed by plateaus. What is that most consistent with?

Episodic changes in natural and human sources (wetland pulses, shifts in fossil fuel output), not one steady source.

Describe methane's growth history since about 1990.

Growth slowed noticeably around 1992, nearly stalled ~1999-2006, then resumed after 2007 and surged in 2020-2021.

Why is methane higher in the Northern Hemisphere?

Most sources (fossil fuel extraction, farming, livestock, northern wetlands) are in the NH, and air takes ~1 year to cross the equator.

Which satellite instrument maps methane daily and can spot large "super-emitter" leaks?

TROPOMI (a trace-gas spectrometer on the European Sentinel-5P satellite), using methane absorption in the shortwave infrared (SWIR) near 2.3 µm.

Near which wavelengths does methane have SWIR absorption features used for leak detection?

About 1.65 µm and 2.3 µm (TROPOMI uses 2.3 µm). Its thermal IR band is near 7.7 µm.

Which gas has SWIR absorption features useful for detecting leaks from oil and gas fields?

Methane (CH4).

What is the chemical name of N2O?

Nitrous oxide (also called laughing gas).

What is atmospheric N2O today versus pre-industrial?

About 337 ppb vs ~270 ppb (about 1.25x), rising ~1 ppb per year.

What are N2O's atmospheric lifetime and 100-year GWP?

Lifetime ~110-120 years. GWP ~270.

What removes N2O from the atmosphere, and why does it last so long?

Almost nothing in the lower air. It is destroyed only in the stratosphere, by UV light and by excited oxygen atoms (a reaction that makes NOx, the nitrogen oxides NO and NO2, which destroy ozone).

What is the largest human source of N2O?

Agriculture: nitrogen fertilizer and manure on soils (about two-thirds of human N2O). A sudden jump in N2O most likely means more fertilizer use.

Which environment emits the most N2O: Arctic tundra, farmland, tropical seas or wetlands?

Farmland (fertilized soils).

Besides warming, what other harm does N2O cause, and how does it rank today?

It destroys stratospheric ozone, and it is now the largest ozone-depleting emission since CFCs were banned.

On an N2O or CO2 graph, why does the monthly curve wiggle above and below the black trend line?

The monthly curve keeps the seasonal cycle, while the trend line has it removed (deseasonalized).

Why is N2O's seasonal cycle tiny (~1 ppb) compared with CO2's (~6-7 ppm)?

Its ~120-year lifetime and weak seasonal sources mean a season's changes are small relative to the large, long-lived total.

N2O has a strong infrared absorption band near what wavelength?

About 4.5 µm (also near 7.8 and 17 µm).

Rank CO2, CH4, N2O, CFCs and SF6 by 100-year GWP, lowest to highest.

CO2 (1) < CH4 (~28-30) < N2O (~270) < CFCs (~5,000-11,000) < SF6 (~23,500-25,000).

Which gas is called the most potent greenhouse gas (highest GWP)?

Sulfur hexafluoride, SF6 (GWP-100 about 23,500-25,000).

What is SF6 used for?

Electrical insulation in high-voltage switchgear.

Which greenhouse gas stays in the atmosphere longest: CO2, SF6, N2O or water vapor?

SF6: about 3,200 years in IPCC tables (a 2017 study suggests closer to ~850 years, still far longer than the others).

Rank CO2, CH4, N2O, CFCs and SF6 by their actual contribution to warming so far.

CO2 > CH4 > N2O ≈ CFCs > SF6 (test keys usually put N2O just above CFCs). Impact = potency × amount, so abundant CO2 dominates despite the lowest GWP.

Why does a longer atmospheric lifetime raise a gas's GWP?

GWP adds up warming over the whole time horizon, so a gas that survives longer keeps absorbing infrared for more of those years.

Which has the largest warming effect per molecule: CO2, CH4, N2O or CFC-12?

CFC-12, roughly 20,000x CO2 per molecule.

Which group of gases has the highest radiative efficiency (warming per added molecule): CO2, or halogenated gases such as CFCs, HFCs and SF6?

Halogenated gases, thousands of times CO2 per molecule. CO2's big total effect comes from its huge abundance, not its efficiency.

What are CFCs, and what were they used for?

Chlorofluorocarbons: human-made gases of chlorine, fluorine and carbon, once used in refrigerators, air conditioners, spray cans and foams. They destroy stratospheric ozone and are strong greenhouse gases.

What makes a gas "halogenated"? Give examples.

It contains halogen atoms (fluorine, chlorine, bromine, iodine): CFCs, HCFCs, HFCs, halons, PFCs, SF6, NF3.

Why are halogenated gases strong greenhouse gases even at parts-per-trillion levels?

They absorb in the 8-12 µm atmospheric window, where CO2 and water vapor barely absorb, and many last decades to thousands of years.

Which lasts longest in the atmosphere: CFC-12, HCFC-22 or HFC-134a?

CFC-12 (~100 years) vs HCFC-22 (~12 years) and HFC-134a (~14 years). CFC-11 lasts ~50 years.

Rank by typical lifetime: water vapor, tropospheric sulfate aerosol, CH4, N2O, SF6.

Water vapor (~9 days) ≈ sulfate aerosol (~1 week) < CH4 (~12 yr) < N2O (~110-120 yr) < SF6 (~850-3,200 yr).