1.2 - Water and carbon change through time

Diurnal and seasonal changes

Short-term variation changes the relative strength of opposing flows even where a system has little net long-term change.

During daylight, incoming solar energy supports evaporation, transpiration and photosynthesis. Plants continue to respire both by day and at night, so their net carbon balance can shift from uptake in sufficient light to release in darkness. Evaporation need not stop at night: stored heat, humidity and wind still matter. Stomata respond to light and water stress, so midday heat can reduce transpiration and carbon uptake if plants close them to conserve water.

Diurnal temperature changes also affect humidity and condensation. Cooling can bring air to its dew point and form dew or fog, transferring water to surface or canopy stores. These inputs can matter locally even if small compared with annual rainfall. A single midday flux measurement cannot represent a full daily budget.

Seasonality changes temperature, rainfall, sunlight and foliage. In a temperate deciduous forest, spring leaf growth increases interception capacity, transpiration and photosynthetic area. Autumn leaf loss reduces those canopy functions and adds litter to the soil. Winter's lower evaporative demand can allow soil recharge and higher runoff where precipitation continues. In a seasonally dry tropical forest, water stress and rainfall timing may be more important than cold in controlling leaf activity. In tundra, snowmelt can precede deep thaw and the brief plant-growth peak.

At large northern-hemisphere scales, seasonal vegetation uptake contributes to atmospheric carbon dioxide falling during the growing season and rising as uptake declines relative to respiration. This is superimposed on the longer-term trend. It is not a universal monthly pattern at every site: air transport, ocean exchange and the distribution of land and vegetation influence the record.

The significance depends on the store and users. A brief flood peak changes water availability and hazard, a dry-season soil deficit constrains growth, and winter respiration can offset summer carbon uptake. Comparing the same season and measuring across full cycles helps distinguish a genuine long-term change from normal timing differences.

Changes over millions of years

Over geological time, changing land positions, relief and sediment burial reorganise both the stores and the pathways of water and carbon.

Organic matter normally decomposes, but a fraction buried in suitable low-oxygen conditions escapes rapid oxidation. Continued burial, pressure and heating can transform some material into fossil-fuel reservoirs over millions of years. Carbonate sediments also accumulate and become rock. Uplift exposes material to weathering, while subduction and metamorphism can contribute to volcanic carbon release. The relative rates of burial, weathering and geological release change atmospheric carbon over long intervals.

Silicate weathering followed by carbonate burial can remove carbon dioxide from the atmosphere–ocean system, while volcanism adds carbon dioxide. Their balance is affected by exposed rock, temperature, water supply and tectonics. Warmer and wetter conditions can increase some weathering rates and oppose warming, but the response is slow and also limited by erosion and fresh-mineral supply. It cannot automatically compensate for a rapid industrial addition over decades.

NASA's 2011 carbon-cycle synthesis describes these fast and slow domains. Their interaction shows why atmospheric carbon is not fixed simply because the Earth's total carbon is approximately conserved. Moving even a small fraction of a huge geological stock into the atmosphere can substantially change the much smaller atmospheric store.

Water distribution also changes. Continental movement and mountain building alter ocean circulation, winds, rain shadows and drainage networks. Climatic cooling transfers water from oceans into long-lived continental ice, lowering sea level; warming reverses that transfer. Over millions of years, volcanic outgassing and exchange with deep Earth further complicate a near-surface closed-cycle approximation. A model appropriate to one year's catchment budget need not include every geological pathway, but its boundary and limits should be stated.

Geological evidence includes sediment sequences, fossil vegetation, mineral indicators, isotopic records and dated landforms. These are interpreted records rather than direct instrument readings of ancient rainfall or atmospheric concentrations. Multiple lines of evidence and age uncertainty matter when connecting an ancient climatic shift to a change in stores.

Research and monitoring methods

Different instruments observe different parts of the cycles, so research combines methods rather than treating one measurement as a complete global budget.

Atmospheric sampling measures gas concentrations, often reported as dry-air mole fractions in parts per million. NOAA's Mauna Loa record began with Scripps measurements in 1958 and separates seasonal variation from a longer-term trend. A single background site is valuable but is not identical to the global surface average. Transport modelling and multiple stations help infer the locations and strengths of carbon sources and sinks.

Flux towers use rapid measurements of wind and gas concentrations to estimate exchanges over an upwind footprint. They can reveal daily and seasonal uptake and release, but the sampled area changes with weather and does not represent every landscape. Separating net exchange into photosynthesis and respiration requires additional assumptions or measurements. Stream gauges measure water level and use a calibrated stage–discharge relationship to estimate flow. Channel change and flows beyond the calibration range increase uncertainty.