In today’s world, CO ₂ concentrations have risen beyond 400 ppm, a level not experienced over the past 800,000 years ¹ . This rise in atmospheric CO ₂ is mostly due to fossil fuel emissions ² and is largely responsible for a 1.5°C increase in air temperatures over land since the 1880s ³ . Together, rising CO ₂ concentrations and temperatures are causing the carbon cycle to experience greater year-to-year perturbations and trends than those experienced during the past deglaciation events ^{4, 5} .

Today, the constituent fluxes and pools of the terrestrial carbon cycle are widely out of equilibrium from pre-historical conditions owing to human activities. For perspective, atmospheric CO ₂ increased from about 180 to 280 ppm since the last glacial period, adding about 220 Pg-C to the atmosphere over a 10,000-year period ⁶ ; this pre-industrial change was associated with a positive trend in the net global carbon exchange rate of about 20 Tg-C y ^-1. In comparison, atmospheric CO ₂ is increasing at a rate of about 4.4 Pg-C y ^-1, as fossil fuel and cement production release 9 +/- 0.5 Pg-C y ^-1, land use change releases 0.9 +/- 0.5 Pg-C y ^-1, and terrestrial ecosystems assimilate 3 +/- 0.5 Pg-C y ^{-1 7} .

Net carbon exchange by terrestrial ecosystems is expected to be variable and changing in our warming and CO ₂-enriched world. This expectation is based on the fact that the rates of photosynthesis are tied to CO ₂ and temperature and that respiration is tied to temperature, photosynthesis, and the size of the carbon pools. From first principles, we know that photosynthesis will increase with CO ₂ concentrations in a diminishing returns fashion, defined by Michaelis-Menten reactions associated with the carboxylation reactions between CO ₂ and ribulose bisphosphate. In addition, increased temperatures accelerate kinetic rates of enzyme reactions, thereby increasing mitochondrial respiration of plants and microbes.

On a year-to-year basis, the secular warming of the Earth’s climate is causing different regions of the world to experience episodes of extremes such as wetness, dryness, hot, and cold, which can perturb the fluxes of carbon to and from the plants and soils of those regions ^{8– 10} . And, on annual to decadal time scales, changes in land use, phenology, greenness of the biosphere, fires, and nitrogen deposition are introducing additional variability and trends on components of the carbon cycle ^{7, 11– 13} .

To answer this question with confidence, we have to separate trends and induced variability from natural variability and random sampling errors. We are entering an era where the sampling record is starting to be long enough to separate signal from noise. We have a 50-year record of CO ₂ concentrations in the atmosphere, providing a top-down constraint on carbon cycle variability ⁷ , and we have a 30-year satellite record, giving us bottom-up information on spatial/temporal variability of the greening of the land ¹³ . Consequently, there has been growing activity to quantify and understand variability of the carbon cycle based on these longer global-scale records. The objective of this review is to survey key literature on the variability of the terrestrial carbon cycle at the global scale published over the past 4+ years (2012 into 2016).

To assess any attribution in the variability of the terrestrial carbon cycle, one must consider the degree of differential and induced modulations of its three major carbon pools. The vegetation, soil, and atmosphere carbon pools have different sizes, different turnover times, and different responses to environmental perturbations, like light, CO ₂ concentration, temperature, and soil moisture ¹⁴ . In other words, there is a disequilibrium between the gains and losses of carbon to and from the plant and soil pools, which can partly be explained by the relatively fast way CO ₂ enters the biosphere via photosynthesis and the relatively slow way it leaves via plant, root, and soil respiration. Consequently, these carbon pools have different susceptibility to anomalous weather and climate variability at global and regional scales. Furthermore, the variability of carbon fluxes is dependent upon changes in such ecological factors as plant functional type, leaf area index, time since disturbance and stand age, nitrogen loading, the intensity and frequency of fires, soil erosion, and transport as dissolved carbon.

Given the superposition of natural (by weather and fire) and human-induced variability (by climate change, increasing CO ₂, changing land use, changing forest age distributions, pollution, and nitrogen deposition) on the carbon cycle ⁸ , can we detect signals or responses among simultaneous variation of numerous drivers? Secondly, are the short-term changes in carbon pools large enough to detect with current observation systems? In assessing if variability in the carbon cycle is occurring, we must consider the detection limit and sampling errors of these systems as they affect how precise or accurate they are.

One group of scientists has reported that the amplitude of the seasonal swing in atmospheric CO ₂ is growing ^{4, 15, 16} . To illustrate this point, we show publicly available data from the long-term monitoring station at Mauna Loa, Hawaii ( Figure 1). This figure shows that the magnitude of the difference between the maximum and minimum values of CO ₂ concentration for each year, after the time series was detrended with a moving filter, has increased by nearly 15% over 55 years. When the global fields of CO ₂, from a network of CO ₂ sampling sites, are considered, it has been deduced that the annual global carbon uptake doubled from 2.46 to 5.06 Pg-C y ^-1 between 1960 and 2010, a rate of 52 Tg-C y ^{-2 5} .

What is the explanation for this temporal increase in CO ₂ amplitude? The answers are manifold. Scientists have reported that this trend is a consequence of the greening of the biosphere ¹³ , stronger northern latitude photosynthesis ^{4, 15} , more photosynthesis by semi-arid ecosystems ^{17, 18} , agriculture and the green revolution ^{16, 19} , tropical temperature anomalies ²⁰ , or increased winter respiration ²¹ .

Evidence for the greening of the terrestrial biosphere is provided by an analysis of satellite remote sensing data. The scientists report that seasonally integrated leaf area index is increasing across a quarter to one-half of the planet’s vegetated area, mostly to the CO ₂ fertilization effect ¹³ . Browning is detected too, but on only 4% of the vegetated land area.

Other scientists point to the northern latitudes, home of the world’s boreal forests, as the locale for a growing carbon sink. Graven et al. ⁴ reported that a 30 to 60% change has occurred in the carbon exchange of boreal forests. Forkel et al. ¹⁵ lend support to this hypothesis by reporting that “the latitudinal gradient of the increasing CO ₂ amplitude is mainly driven by positive trends in photosynthetic carbon uptake caused by recent climate change and mediated by changing vegetation cover in northern ecosystems”. Others claim winter conditions may have a strong influence on year-to-year variations in the growth of the CO ₂ concentration amplitude. Yu et al. ²¹ argue that warmer winters have less snow, which causes soil temperatures to be colder, reducing winter respiration. They conclude that this mechanism explains 25% of the enhancement of the carbon sink across boreal forests.

In addition, not all scientists agree on whether the activity of boreal forests explains the variability of the terrestrial carbon cycle. One team ²⁰ reported that 50% of the interannual variation of the CO ₂ growth rate, between 1959 and 2011, was associated with tropical air temperature; a 1°C anomaly in tropical air temperature corresponded with a 3.5 +/- 0.6 Pg-C y ^-1 anomaly in the CO ₂ growth rate. Others attribute the trends in the global carbon sink to the growth of semi-arid vegetation ^{17, 18} ; semi-arid ecosystems are experiencing a 0.04 Pg-C y ^-2 trend in their carbon sink, which is about 57% of the global trend. Two other groups point to the role of agriculture as a modulating factor of the global CO ₂ concentration record. Gray et al. ¹⁶ and Zeng et al. ¹⁹ attribute a significant part of the increase in the CO ₂ seasonal amplitude (17 to 25%) to the agricultural green revolution; cereal production in the northern hemisphere increased by 240 percent over the 47-year period between 1961 and 2008, thereby increasing the net carbon uptake of crops by 0.33 Pg-C.

To better understand how and why the terrestrial carbon cycle is experiencing variability, let’s examine the potential for modulation and or trends in the distinct plant and soil carbon pools.

The variability of the global carbon cycle is changing, as the amplitude of seasonal CO ₂ concentration and the net land sink are increasing. The causes of these trends in the global carbon cycle remain uncertain and a challenge for future research.

In this review, we show that different analyses have different explanations for why the seasonal CO ₂ amplitude and land sink are increasing. We advise investigators of future studies to test whether or not reported trends are significantly different from random errors associated with the uncertainties in modeling and measuring gross and net carbon fluxes.

Photosynthesis is the starting point of the carbon cycle. Yet, despite decades of research, our ability to produce an estimate of global photosynthesis with narrow confidence intervals and high accuracy remains elusive. Only with more accurate estimates of photosynthesis will we be able to better resolve and understand differences between data-driven and process models and to better simulate responses between net and gross carbon fluxes to temperature, precipitation, CO ₂, and nitrogen.

The response of global photosynthesis to CO ₂ fertilization remains unresolved, as the variations in the assimilation fluxes are small relative to the uncertainties of the drivers and information systems, using bottom-up and top-down methods. Indirect effects can be important too. For example, lengthening growing seasons can increase gross primary productivity on regional scales ⁴³ . Vegetative regrowth may be offsetting losses of carbon by preceding fires. Thawing of the permafrost may expose and release vast stores of carbon that have been decoupled for millennia, while warming northlands are getting greener. Many experiments show acclimation effects of respiration to temperature ⁴⁴ . This is a process that is not well considered by models, though some are starting to consider this factor ³⁰ .

In closing, we need to continue the collection of long-term and multi-faceted products that are used to assess the global carbon cycle. Additional data from new satellites and longer time series from eddy covariance and CO ₂ concentration networks will reduce uncertainty in sampling and modeling and improve our ability to answer the questions associated with carbon cycle variability and trends.