\section{Conclusions}
\label{sec:Conclusions}
With an increasing production of plastic goods, the environmental concerns are rapidly growing. Plastic debris in the ocean is a known threat for marine life and causes economic loss. We predict an increase in total plastic debris entering the ocean (P\textsubscript{O}) from 13.4 Mt in 1990 to 18.7 Mt in 2015 (+40\%). The ten countries with the highest impact on P\textsubscript{O} account for 15.1 Mt in 2015 which represents 81\% of the global P\textsubscript{O}. We differ between inputs from riverine and coastal sources that account for 6\% and 94\% of P\textsubscript{O} respectively. In the time period 1990-2015, the riverine input has more than doubled (+134\%). Geographically, the strongest increase in P\textsubscript{O} can be seen in Asia (+4.3 Mt) and economically, middle-income countries (+4.8 Mt) show the strongest increase. In 2015, 84\% of P\textsubscript{O} (15.7 Mt) originated from Asia and 90\% of P\textsubscript{O} (16.8 Mt) originated from middle-income countries (representing 58\% of the world's population). We predict that from the 18.7 Mt of P\textsubscript{O} in 2015, 60\% (11.2 Mt) originate from losses in the waste collection system and 40\% (7.5 Mt) originate from waste that was not collected. Thus, measures which target an improved waste collection and treatment system in Asia have the potential to reduce the global P\textsubscript{O} by up to 8.4 Mt (45\% of P\textsubscript{O}) in an ideal scenario. Already minor improvements could have a significant impact on the global P\textsubscript{O}. These results imply that a reduction of plastic waste is a more effective measure than ocean clean-ups to reduce P\textsubscript{O}. Besides the period 1990-2015, we also created future estimates for 2030 and 2050. We predict that the waste production of African and Asian countries will increase, which will also further increase P\textsubscript{O} to 19.7 Mt in 2030 and 21.6 Mt in 2050. Besides  P\textsubscript{O} we also estimated the plastic waste entering the environment (P\textsubscript{E}). We predict an increase from 31.8 Mt in 1990 to 51.0 Mt in 2015 (+60\%). The faster growth of P\textsubscript{E} compared to P\textsubscript{O} can be explained by an increase of plastic waste production in regions without direct access to the sea. P\textsubscript{E} in these regions can only reach the ocean by riverine transport.\\
We used a NNET and RF model to predict the amounts of produced municipal solid waste using the predictor variables \textit{urban population}, \textit{greenhouse gas} and \textit{young population}. The RF model has an error of prediction of 2\% for high-income countries. This error increases for middle (12\% for upper and 16\% for lower) and low-income countries (86\%). The NNET model is more erroneous in 103 out of 106 countries and shows the same trend regarding prediction error and the country's income as the RF model.\\
High errors might arise due to the quality of the predictor variables, missing data, biases in the training data or overfitting of the model. To increase the overall prediction accuracy, we propose to create multiple models for the income classes and to account for autocorrelation in the time-series data. Furthermore, a choice of less complex models (e.g. multiple linear regression) could help to increase the prediction performance. A focus to reduce the prediction error should be laid on the countries that contribute most to P\textsubscript{O}. Additionally, we want to highlight the need for more reliable data on the quantity and composition of each country's waste, as well as of the composition of the waste management system, the estimated waste collection rates and losses from the collection system. The recently published report \textit{"What a waste 2.0: A global Snapshot of Solid Waste Management to 2050"} \cite{Kaza2018} (published on the 20\textsuperscript{th} September 2018 and thus not included here) provides an updated and extended version of our data and might be useful to obtain such data.\\
We were able to show that modelling the waste production of each country in the time period 1990-2015 is possible using socio-economic predictors. While the errors of the estimates could be reduced by increasing the number of models (e.g. for each income class) and modelling techniques (e.g. multiple linear regression), our results are still comparable to similar studies. We show that reducing losses from the waste treatment system and increasing the fraction of waste collection in Asian countries could significantly reduce the plastic entering the ocean. Our results help to gain further knowledge on the spatio-temporal distribution of P\textsubscript{O}. This helps to better understand the \textit{missing plastic problem} as well as to better predict and quantify impacts of plastic waste on both the marine environment and the economy (e.g. tourism or the fishing industry). Furthermore, the results improve our understanding of waste flows from the municipal level via the waste collection and treatment system to the environment and, finally, to the oceans. Understanding the waste flows is a crucial step to determine efficient actions of P\textsubscript{E} and P\textsubscript{O} prevention. This can be help to reduce P\textsubscript{O} by target-oriented measures or sanctions on specific parts of the waste flow (e.g. collection rates or losses). Additionally, our work will help to create improved models to predict the waste production or P\textsubscript{E} and P\textsubscript{O}. However, there are still various open questions. A better knowledge on the drivers of municipal solid waste generation could help to better predict the production of waste. Furthermore, more detailed insights on the waste collection and treatment on a national and regional level cold help to better understand the pathways of (plastic) waste and pinpoint losses from the system. Both could help to reduce the plastic waste that enters the environment and the oceans in the future. Additionally, a better understanding of the degradation processes of plastic in the marine environment can help to understand the fate and sinks of the marine plastic debris that already entered our oceans. Both prevention and mitigation of marine plastic debris are essential to protect the marine environment from the deleterious effects of plastic waste.