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The Amazon rainforest holds more than 40% of all remaining tropical rainforest and is a key component of the climate system. The scale of deforestation in the Amazon significantly impacts both local and global climates. Under a business-as-usual scenario as much as 40% of the Brazilian Amazon rainforest will be lost by 2050. Despite the magnitude of these changes and its importance, the overall effects of deforestation on rainfall remain uncertain. Land-use change influences rainfall through a variety of mechanisms acting at local to continental scales. As such, previous research indicates conflicting responses to rainfall depending on the scales studied. In reality, rainfall processes interact across these scales, but until recently have been impossible to capture within a single model due to computational expense. Consequently we are unable to rely on these simulations as future estimations of rainfall for such a sensitive and anthropogenically impacted region as the Amazon. In this study, we overcome these limitations by running convection permitting simulations (horizontal resolution 4.5km) over a large domain (6000km covering the majority of South America) using a Tropical configuration of the UK Met Office Unified Model. The high-resolution and continental-scale of these simulations present an opportunity to reduce the uncertainty in Amazonian rainfall estimates within a single model and ensures rainfall processes and interactions across scales are captured. To investigate the impacts of deforestation we will include a series of land-use sensitivity runs making use of a range of socioeconomic scenarios to 2050. Here we present initial results from our simulations, indicating how localised storms, mesoscale convective systems and large-scale circulations respond to land-use change.

Increasingly, we are using high-resolution convection-permitting models for climate projections but these models are less well understood in terms of the interaction between soil moisture, precipitation and evapotranspiration. The work was motivated by the discovery of warm, dry biases in summer in the 2.2 km convection-permitting model over France and eastern Europe compared to the 12 km convection-parametrised model that were associated with drier soils. We analyse several 12 km and 2.2 km versions of the Met Office Unified Model including sensitivity tests relating to soil hydraulics, land cover type and runoff model. We conduct similar tests using the land surface only to compare results between online and offline versions as the absence of some feedbacks can also produce differences.

We present a detailed analysis of mechanisms underlying the evapotranspiration response to increased $$\hbox {CO}_2$$ in HadGEM2-ES, focussed on western Amazonia. We use three simulations from CMIP5 in which atmospheric $$\hbox {CO}_2$$ increases at 1% per year reaching approximately four times pre-industrial levels after 140 years. Using 3-hourly data, we found that evapotranspiration (ET) change was dominated by decreased stomatal conductance ( $$g_s$$ ), and to a lesser extent by decreased canopy water and increased moisture gradient (specific humidity difference between surface and near-surface). There were large, non-linear decreases in ET in the simulation in which radiative and physiological forcings could interact. This non-linearity arises from non-linearity in the conductance term (includes aerodynamic and stomatal resistance and partitioning between the two, which is determined by canopy water availability), the moisture gradient, and negative correlation between these two terms. The conductance term is non-linear because GPP responds non-linearly to temperature and GPP is the dominant control on $$g_s$$ in HadGEM2-ES. In addition, canopy water declines, mainly due to increases in potential evaporation, which further decrease the conductance term. The moisture gradient responds non-linearly owing to the non-linear response of temperature to $$\hbox {CO}_2$$ increases, which increases the Bowen ratio. Moisture gradient increases resulting from ET decline increase ET and thus constitute a negative feedback. This analysis highlights the importance of the $$g_s$$ parametrisation in determining the ET response and the potential differences between offline and online simulations owing to feedbacks on ET via the atmosphere, some of which would not occur in an offline simulation.

This article reviews recent scientific progress, relating to four major systems that could exhibit threshold behaviour: ice sheets, the Atlantic meridional overturning circulation (AMOC), tropical forests and ecosystem responses to ocean acidification. The focus is on advances since the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). The most significant developments in each component are identified by synthesizing input from multiple experts from each field. For ice sheets, some degree of irreversible loss (timescales of millennia) of part of the West Antarctic Ice Sheet (WAIS) may have already begun, but the rate and eventual magnitude of this irreversible loss is uncertain. The observed AMOC overturning has decreased from 2004–2014, but it is unclear at this stage whether this is forced or is internal variability. New evidence from experimental and natural droughts has given greater confidence that tropical forests are adversely affected by drought. The ecological and socio-economic impacts of ocean acidification are expected to greatly increase over the range from today’s annual value of around 400, up to 650 ppm CO2 in the atmosphere (reached around 2070 under RCP8.5), with the rapid development of aragonite undersaturation at high latitudes affecting calcifying organisms. Tropical coral reefs are vulnerable to the interaction of ocean acidification and temperature rise, and the rapidity of those changes, with severe losses and risks to survival at 2 °C warming above pre-industrial levels. Across the four systems studied, however, quantitative evidence for a difference in risk between 1.5 and 2 °C warming above pre-industrial levels is limited.

Background: The forests of north-west Amazonia are characterised by the highest levels of tree diversity in the world, high rainfall and relatively fertile soils. Aims: Here we present a comprehensive description of the carbon cycle of two 1 ha forests plots in Allpahuayo, near Iquitos, Peru, one on an occasionally inundated alisol/gleysol landscape, the other on an arenosol (sandy soil). Methods: Data on the components of net primary productivity (NPP) and autotrophic respiration were collected over the period 2009–2011, and summed to estimate gross primary productivity (GPP) and carbon use efficiency (CUE). Results: Overall, these forests showed high values of GPP (39.05 ± 4.59 and 41.88 ± 4.60 Mg C ha−1 year−1). Despite the lack of a dry season, the forests showed distinct seasonality in tree growth, litterfall, flowering and fine root productivity. This showed that tropical forests with little seasonality in water supply can still exhibit distinct seasonality in NPP and carbon use, apparently synchron...

Background: Terra preta do indio or ‘dark earth’ soils formed as a result of a long-term addition of organic matter by indigenous peoples in Amazonia. Aims: Here we report on the first study of productivity, allocation and carbon cycling from a terra preta plot in eastern Amazonia (Caxiuana, Para, Brazil), and contrast its dynamics with a nearby plot on infertile soil (ferralsols). Methods: We determined total net primary production (NPP) for fine roots, wood, and canopy and total autotrophic respiration (rhizosphere, wood, and canopy respiration) from two 1-ha plots on contrasting soils. Results: Both gross primary productivity (GPP) (35.68 ± 3.65 vs. 32.08 ± 3.46 Mg C ha−1 year−1) and carbon use efficiency (CUE) (0.44 ± 0.06 vs. 0.42 ± 0.05) were slightly higher at the terra preta plot. Total NPP (15.77 ± 1.13 Mg C ha−1 year−1 vs. 13.57 ± 0.60 Mg C ha−1 year−1) and rates of fine root production (6.41 ± 1.08 vs. 3.68 ± 0.52 Mg C ha−1 year−1) were also greater at the terra preta plot vs. the tower plot. C...

In Paris, France, December 2015, the Conference of the Parties (COP) to the United Nations Framework Convention on Climate Change (UNFCCC) invited the Intergovernmental Panel on Climate Change (IPCC) to provide a "special report in 2018 on the impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways". In Nairobi, Kenya, April 2016, the IPCC panel accepted the invitation. Here we describe the response devised within the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) to provide tailored, cross-sectorally consistent impacts projections. The simulation protocol is designed to allow for (1) separation of the impacts of historical warming starting from pre-industrial conditions from other human drivers such as historical land-use changes (based on pre-industrial and historical impact model simulations); (2) quantification of the effects of additional warming up to 1.5 °C, including a potential overshoot and long-term effects up to 2299, compared to a no-mitigation scenario (based on the low-emissions Representative Concentration Pathway RCP2.6 and a no-mitigation pathway RCP6.0) with socio-economic conditions fixed at 2005 levels; and (3) assessment of the climate effects based on the same climate scenarios but accounting for simultaneous changes in socio-economic conditions following the middle-of-the-road Shared Socioeconomic Pathway (SSP2, Fricko et al., 2016) and differential bio-energy requirements associated with the transformation of the energy system to comply with RCP2.6 compared to RCP6.0. With the aim of providing the scientific basis for an aggregation of impacts across sectors and analysis of cross-sectoral interactions that may dampen or amplify sectoral impacts, the protocol is designed to facilitate consistent impacts projections from a range of impact models across different sectors (global and regional hydrology, global crops, global vegetation, regional forests, global and regional marine ecosystems and fisheries, global and regional coastal infrastructure, energy supply and demand, health, and tropical cyclones).

We present a method to derive the vertical-wind field at a scale of 1–2 km from a high-resolution Doppler radar, by performing vertical scans in the plane of the mean wind and tracking turbulent features on a scale of several hundred metres in the radial-velocity field between consecutive scans. The method is applied to a line of cumulonimbus clouds observed by the high-resolution Chilbolton radar in southern England. The storm was not a squall line, since it was propagating parallel rather than perpendicular to its length. A sequence of scans was taken over an hour-long period; from these scans we derive the u and w components of the wind. Updraughts of up to 15 m s−1 are found. From these observations, the mass flux of a ‘typical’ storm cell was estimated: over the 50 min lifetime of the cell, the total mass of air transported from the boundary layer to the upper troposphere was found to be around 50 megatonnes. The profile of hour-averaged momentum flux against height was then calculated and compared to the vertical shear of the mean horizontal wind. It was found that above 4.5 km, the momentum flux was down-gradient, with an equivalent eddy diffusivity of around 103 m2s−1. Below this height, however, the flux was counter-gradient, with an equivalent eddy diffusivity of around − 103m2s−1. We propose a mechanism to explain this behaviour, involving the prevalence of precipitation cooling and descent on the downshear side of convergence lines. A synthesis of the major wind flows in the storm reveals that, despite superficial appearance, the storm did not fit the usual multi-cell paradigm, in which gust fronts play a key roll in triggering new cells downwind. Rather, the convergence lines associated with the convection were more akin to warm frontal surfaces that remain tied to the base of each cell, new cells tending to form upwind of older cells. Copyright © 2008 Royal Meteorological Society

Background: The forests of western Amazonia are known to be more dynamic that the better-studied forests of eastern Amazonia, but there has been no comprehensive description of the carbon cycle of a western Amazonian forest. Aims: We present the carbon budget of two forest plots in Tambopata in south-eastern Peru, western Amazonia. In particular, we present, for the first time, the seasonal variation in the detailed carbon budget of a tropical forest. Methods: We measured the major components of net primary production (NPP) and total autotrophic respiration over 3–6 years. Results: The NPP for the two plots was 15.1 ± 0.8 and 14.2 ± 1.0 Mg C ha−1 year−1, the gross primary productivity (GPP) was 35.5 ± 3.6 and 34.5 ± 3.5 Mg C ha−1 year−1, and the carbon use efficiency (CUE) was 0.42 ± 0.05 and 0.41 ± 0.05. NPP and CUE showed a large degree of seasonality. Conclusions: The two plots were similar in carbon cycling characteristics despite the different soils, the most notable difference being high allocation ...

Background: The dry transitional forests of the southern Amazonia have received little attention from a carbon cycling and ecosystem function perspective, yet they represent ecosystems that may be impacted by global climate change in the future. Aims: To compare the full carbon cycle for two 1-ha forest plots that straddle the ecotone between humid forest and dry forest in Amazonia, ca. 100 km from Santa Cruz, Bolivia. Methods: 2.5 years of measurements of the components of net primary production (NPP) and autotrophic respiration were collected. Results: Total NPP was 15.5 ± 0.89 Mg C ha−1 year−1 at the humid site and 11.27 ± 0.68 Mg C ha−1 year−1 at the dry site; a total Gross Primary Production (GPP) of 34.14 ± 2.92 Mg C ha−1 year−1 and 26.88 ± 2.70 Mg C ha−1 year−1 at the two sites. Carbon use efficiency for both sites was higher than reported for other Amazonian forests (0.45 ± 0.05 and 0.42 ± 0.05). Conclusions: Drier soil conditions selected for the dry deciduous tree species which had higher leaf photosynthesis and total GPP. NPP allocation patterns were similar at the two sites, suggesting that in terms of carbon allocation, the dry forests of the southern Amazonia behave as a scaled-down version of wetter humid forests.

Background: The impact of fire on carbon cycling in tropical forests is potentially large, but remains poorly quantified, particularly in the locality of the transition forests that mark the boundaries between humid forests and savannas. Aims: To present the first comprehensive description of the impact of repeated low intensity, understorey fire on carbon cycling in a semi-deciduous, seasonally dry tropical forest on infertile soil in south-eastern Amazonia. Methods: We compared an annually burnt forest plot with a control plot over a three-year period (2009–2011). For each plot we quantified the components of net primary productivity (NPP), autotrophic (Ra) and heterotrophic respiration (Rh), and estimated total plant carbon expenditure (PCE, the sum of NPP and Ra) and carbon-use efficiency (CUE, the quotient of NPP/PCE). Results: Total NPP and Ra were 15 and 4% lower on the burnt plot than on the control, respectively. Both plots were characterised by a slightly higher CUE of 0.36–0.39, compared to evergreen lowland Amazon forests. Conclusions: These measurements provide the first evidence of a distinctive pattern of carbon cycling within this transitional forest. Overall, regular understorey fire is shown to have little impact on ecosystem-level carbon fluxes.

[1] Tropical montane regions present a complex local climate but one that may be very sensitive to local and global change. Therefore, it is important to assess their current climatological state, and to understand how the large-scale circulation may affect local-scale cloud patterns. We examine the cloud climatology of a tropical Andean montane region in the context of tropical South American climate in terms of seasonal/diurnal cycles using a corrected ISCCP (International Satellite Cloud Climatology Project) DX cloud product (1983–2008), MODIS (Moderate Resolution Imaging Spectroradiometer) MOD35 visible cloud flags (2000–2008) and ground-based cloud observations. Cloud climatologies were compared for three elevation zones: highlands (puna grassland), eastern slope (the montane forest) and lowlands. We found that in the dry season (JJA) the study area is part of a localized region of higher cloud frequency relative to other parts the eastern slope, and also relative to the adjacent highlands and lowlands. The highlands exhibited the greatest amplitude mean annual cycle of cloud frequency, with a minimum in June for all times of day. There were contrasts between the three zones with regard to the month in which the minimum cloud frequency occurs between different times of day. Higher lowland and eastern slope cloud frequencies compared with those on the puna in the early hours in the wet season suggest low-level convergence at lower elevations. Comparisons between satellite products show that ISCCP and MODIS produce very similar annual cycles although the absolute cloud frequencies are higher in ISCCP data.

[1] The climate and ecology of tropical montane systems is intimately connected with the complex spatial dynamics of cloud occurrence, but there have been few studies of the patterns and trends of cloud occurrence in tropical montane regions. We examine trends and variability in the cloud climatology of the Andes/Amazon transition in SW Amazonia using satellite data and ground-based observations. Results were compared for three zones within the study area: highlands (puna grassland), eastern slope (Tropical Montane Cloud Forest or TMCF) and lowlands. Time series of cloud frequency from ISCCP (International Satellite Cloud Climatology Project) were correlated with sea surface temperature (SST) anomalies from the HadISST data set for 5 regions including the tropical North Atlantic and the tropical Pacific. Detrended lowland cloud frequencies were significantly correlated with detrended tropical North Atlantic SSTs in the late dry season (August/September), whereas the eastern slope and the highlands were not significantly correlated with tropical North Atlantic SSTs. Pacific SST correlations were highest for eastern slope and highlands from March to May. Indian Ocean SST anomalies were significantly correlated with dry season cloud frequency for the lowlands and highlands. There are significant decreasing trends in cloud frequency on the lowlands in January, March and September and in March on the eastern slope. Trends in sunshine duration, 850 hPa zonal winds over the central Amazon, increases in diurnal temperature range, and comparisons with MODIS (Moderate Resolution Imaging Spectroradiometer) and observational data support the existence of these trends, and a link with the increasing trend in tropical North Atlantic SSTs. We suggest that continued increases in tropical North Atlantic SSTs will further reduce cloud frequency in the lowlands adjacent to the TMCF in the late dry season at least. In addition, a future increase in the occurrence of El Nino events would lead to decreased cloud frequency on the eastern slope and highlands.

Tropical rainforests in Africa are one of the most under-researched regions in the world, but research in the Amazonian rainforest suggests potential vulnerability to climate change. Using the large ensemble of Atmosphere-only general circulation model (AGCM) simulations within the weather@home project, statistics of precipitation in the dry season of the Congo Basin rainforest are analysed. By validating the model simulation against observations, we could identify a good model performance for the June, July, August (JJA) dry season, but this result does need to be taken with caution as observed data are of poor quality. Additional validation methods have been used to investigate the applicability of probabilistic event attribution analysis from large model ensembles to a tropical region, in this case the Congo Basin. These methods corroborate the confidence in the model, leading us to believe the attribution result to be robust. That is, that there are no significant changes in the risk of low precipitation extremes during this dry season (JJA) precipitation in the Congo Basin. Results for the December, January, February dry season are less clear. The study highlights that attribution analysis has the potential to provide valuable scientific evidence of recent or anticipated climatological changes, especially in regions with sparse observational data and unclear projections of future changes. However, the strong influence of sea surface temperature teleconnection patterns on tropical precipitation provides more challenges in the set up of attribution studies than midlatitude rainfall.