By Kenneth Richard
Satellite observations indicate the Earth has become much greener in recent decades. According to scientists, the overwhelming majority of the “significant increases in tropical forests and the forests of North America, Eurasia, and China” since the early 1990s can be attributed to the combination of CO2 fertilization (56%) and climate change (35%).
Image Source: O’Sullivan et al., 2019
“The recent warming hiatus (1998–2013) was identified as a potential key mechanism behind the increased land sink during this period via reduced ecosystem respiration (Ballantyne et al., 2017).”
“At the global scale, simulated NPP [net primary production, greening] increased substantially over the 20th century to present day from 56.2 (mean of 1901–1910) to 66.0 Pg C/year (mean of 2007–2016) with positive contributions from all drivers considered, including rising CO2 concentrations (referred to as CO2 fertilization), nitrogen deposition, climate, and carbon‐nitrogen as well as carbon‐climate synergies. The relative contribution of these drivers to this overall NPP increase amounts to 60% for increased CO2, 15% for nitrogen deposition, 8% for carbon‐nitrogen synergy, 9% for carbon‐climate synergy, and 8% for climate. Both CO2 fertilization and nitrogen deposition individually caused a smooth, transient increase in NPP, in line with the trajectory of the corresponding drivers.”
“[R]esults show a global NPP [net primary production, greening] increase of 3.4 Pg C/year between the early 1990s (mean of 1990–1996) and the end of our study period (2010–2016), with CO2 fertilization and climate being the dominant drivers, accounting for 56% and 35% of the overall change, respectively.”
“Carbon‐climate interactions led to significant increases in tropical forests and the forests of North America, Eurasia, and China.”
A dozen new papers attest to the substantially positive impact that CO2 fertilization and warming has had on the biosphere.
• Due especially to the rise in CO2 concentrations, 52% of the globe’s vegetated lands have shown statistically significant greening/gross primary production trends since 1981, whereas just 12% of vegetated areas have been browning. CO2’s greening effect has been underestimated by 60% with outdated models.
Image Source: Winkler et al., 2019
“Historical increase of atmospheric CO2 concentration, from 280 to current 400 ppm, has resulted in enhanced GPP [gross primary production/greening] due to its radiative and physiological effects, which is indirectly evident in amplified seasonal swings of atmospheric CO2 concentration and large scale increase in summer time green leaf area. Thus, these observables, expressed as sensitivities to ambient CO2 concentration, might serve as predictors of changes in GPP and help to reduce uncertainty in multimodel projections of terrestrial carbon cycle entities. This study is focused on the northern high latitudes (NHL, north of 60°N) where significant and linked changes in climate and vegetation have been observed in the past 3–4 decades: 52% of the vegetated lands show statistically significant greening trends over the 36-year record of satellite observations (1981–2016, Methods), while only 12% show browning trends, mostly in the North American boreal forests due to disturbances.”
“Here, we apply the concept of Emergent Constraints (EC) to reduce uncertainty in multi-model projections of GPP using historical simulations and satellite observations of LAI focusing on NHL. We find that the EC estimate [of the effect of CO2 on greening/GPP] is 60% larger than the commonly accepted multi-model mean value, in line with a recent study that assessed the impact of physiological effects of higher CO2 concentration on GPP of northern hemispheric extra-tropical vegetation. Detailed independent analyses of insitu CO2 measurements and atmospheric inversions imbue confidence in our conclusions. Our central finding is, the effect of ambient CO2 concentration on terrestrial photosynthesis is larger than previously thought, and thus, has important implications for future carbon cycle and climate.”
• According to observations, the Earth – especially dryland regions – has been greening due to climate change (CO2 and temperature rise).
“Recent Earth observation studies find a greening of the Earth and in particular in global drylands, which is commonly interpreted as a global increase in net primary production and has been attributed to climate change. Although changes in rainfall, fire regimes, elevated temperatures, atmospheric CO2 and nitrogen depositions are suggested explanations, only few studies provide quantitative evidence on both the biophysical processes (changes in vegetation cover, structure and composition) and controlling factors of long-term dryland vegetation trends.”
“Our results show that both net primary production (NPP) and heterotrophic respiration (HR) of northern peatlands increased over the past century in response to CO2 and climate change.”
• The “remarkable” vegetation greening in the Yellow River Basin since 2000 is expected to continue for 73% of the region.
Image Source: Wang et al., 2019
“Changes in Vegetation Greenness in the Upper and Middle Reaches of the Yellow River Basin over 2000–2015 … In this study, the vegetation dynamic characteristics were analyzed for unconverted forestland, shrubland, grassland, cropland, and converted forestland, shrubland, and grassland from cropland over 2000–2015 in the upper and middle reaches of the Yellow River. … The results obtained were as follows: (1) Vegetation greening was remarkable in the entire study region (0.036 yr−1).”
“Overall greening trend in the upper and middle reaches of the Yellow River indicated great achievements have been obtained since the implementation of the GTGP. Vegetation restoration exerted stronger influences on converted types from cropland than unconverted types. In the future, approximately 73.1% of the study region is expected to continue increasing [greening].”
• Water-use efficiency – the ability for plants to compensate for water loss – has improved directly due to “rising atmospheric CO2 and contemporary climate change.”
“Human-caused CO2 emissions over the past century have caused the climate of the Earth to warm and have directly impacted on the functioning of terrestrial plants. We examine the global response of terrestrial gross primary production (GPP) to the historic change in atmospheric CO2. The GPP of the terrestrial biosphere has increased steadily, keeping pace remarkably in proportion to the rise in atmospheric CO2. Water-use efficiency, namely the ratio of CO2 uptake by photosynthesis to water loss by transpiration, has increased as a direct leaf-level effect of rising CO2. This has allowed an increase in global leaf area, which has conspired with stimulation of photosynthesis per unit leaf area to produce a maximal response of the terrestrial biosphere to rising atmospheric CO2 and contemporary climate change.”
• A “large increase” in total biomass and improvement in water use efficiency is assessed for the Amazon region under elevated CO2 (700 ppm). Elevated CO2 “nullified” the effect of drought.
“The large increase in total biomass and the substantial improvement in WUEP [water use efficiency] under eCO2 [elevated CO2, 700 ppm], and the sharp decline in leaf area under water stress widen our knowledge on the physiology of this important species for the forest management of large areas in the Amazon region.”
“Climate models predict an increase in atmospheric CO2 concentration and prolonged droughts in some parts of the Amazon, but the effect of elevated CO2 is still unknown. Two experiments (ambient CO2 ‒ 400 ppm and elevated CO2 ‒ 700 ppm) were conducted to assess the effect of drought (soil at 50% field capacity) on physiological parameters of Carapa. At ambient CO2 concentration, light-saturated net photosynthetic rate (PNsat) was reduced by 33.5% and stomatal conductance (gs) by 46.4% under drought, but the effect of drought on PNsat and gs was nullified at elevated CO2. Total plant biomass and leaf area production were also reduced (42‒47%) by drought. By changing leaf traits, Carapa is able to endure drought, as the consumptive use of water was reduced under drought (32‒40%). The improvement of PNsat under elevated CO2 and water stress and the leaf plasticity of Carapa broaden our understanding of the physiology of Amazonian trees.”
• Plants grown under elevated CO2 have “higher biomass, plant height, and leaf area.” Elevated CO2 “may mitigate the negative effects of water deficit” in soybeans.
“In this study, we evaluated the individual and combinatory effects of E[CO2] [elevated CO2] and water deficit on the physiology and root molecular responses in soybean. Plants growing under E[CO2] [elevated CO2] exhibited increased photosynthesis that resulted in a higher biomass, plant height, and leaf area. E[CO2] decreased the transcripts levels of genes involved in iron uptake and transport, antioxidant activity, secondary metabolism and defense, and stress responses in roots. When plants grown under E[CO2] [elevated CO2] are treated with instantaneous water deficit, E[CO2] reverted the expression of water deficit-induced genes related to stress, defense, transport and nutrient deficiency. Furthermore, the interaction of both treatments uniquely affected the expression of genes. Both physiological and transcriptomic analyses demonstrated that E[CO2] may mitigate the negative effects of water deficit on the soybean roots.”
• Elevated temperature and CO2 increased maize and soybean yield by 25%-31%.
“Maize had 25% yield increase under elevated temperature (eT). Soybean had 31% yield increase under elevated CO2 (eCO2) with eT. Elevated temperature with and without eCO2 increased grain oil concentrations.”
• Rice yield is “significantly higher” with elevated CO2.
“Climate change associated with rising atmospheric carbon dioxide (CO2) concentration may have impact on crop production and soil health. Increase in atmospheric CO2 concentration may enhance crop growth with higher demand for nutrients by the crop. An experiment was conducted during July-October, 2013 using Free Air Carbon Dioxide Enrichment facility at the Indian Agricultural Research Institute, New Delhi to study the impact of elevated CO2 and nitrogen (N) dose on growth, yield and nitrogen uptake in rice crop. Four doses of N, i.e., control, 0.6 g N pot-1 (75% recommended dose of N), 0.8 g N pot-1 (100% recommended dose of N) and 1.0 g N pot-1 (125% recommended dose of N) were applied in both ambient (395 ppm) and elevated CO2 (550±20 ppm) conditions. Grain and biomass yield of rice was significantly higher under elevated CO2 condition. Plant growth and yield parameters also increased with increased N doses in both elevated and ambient CO2 conditions. Nitrogen concentration of grain and straw decreased under high CO2 level but N uptake increased under elevated CO2 condition. Agronomic efficiency of N was higher under elevated CO2 while recovery efficiency of N remained unaffected. The study showed that although yield of rice increases under elevated CO2 condition, to maintain plant nitrogen concentration, application of additional dose of N is required.”
• Trees “may temporally benefit from warming climate” in Asian boreal forests.
“While summer GST [ground surface temperature] had a somewhat consistently positive correlation with tree growth, winter GST has shifted from a negative to a strongly positive correlation with growth in the last decade, coincidental with a sharp increase in winter GST since 2004. Winter GST is also strongly correlated with the rapidly thawing permafrost dynamics. Overall, our results suggest a link between recent changes in the permafrost and shifts in climate‐growth correlations for one of the main boreal tree species. As a result, L. gmelinii has experienced an important increase in radial growth that may indicate that, unlike what has been reported for other boreal species, it may temporally benefit from warming climate in the continuous permafrost region of the Asian boreal forests.”
• Due to climate change, the height growth for oak trees has been “significantly higher” during the last 30-35 years than the decades prior.